Seeing the check engine light come on can be worrying. Even if your car seems to be running normally, that light can make you think something is really wrong.
But while it might look bad, a check engine light when your car is fine often isn’t a big problem.
In this helpful guide, we’ll talk about some common reasons for the confusing check engine light. We’ll help you figure out if it’s okay to keep driving or if you need to take your car to the repair shop.
So, why check engine light comes on but car runs fine? The check engine light on but car runs fine is commonly caused by a clogged air filter, bad MAF sensor, or bad oxygen sensors. It indicates a problem with the engine, which can be simple or complex. To identify the main cause, use an OBDII scanner to retrieve the code instantly. Additionally, a loose or faulty gas cap can trigger the check engine light. If the check engine light is blinking or flashing, it indicates an engine misfire, which requires immediate attention as it can lead to severe engine damage.
Here I’ll take you through the most common causes due to which check engine light might come on, and what you can do about them.
|Check Engine Light Orange or Yellow and Is Steady||Check Engine Light Red and Blinking/Flashing|
|Bad or loose gas cap||Engine misfire|
|Bad EGR valve||Bad spark plugs and ignition coil|
|Leaks in EVAP system||Clogged fuel injector|
|Malfunctioning O2 and MAF sensor||Vacuum leaks after MAF sensor|
Note: The information I presented in this article is gathered from online automotive forums, OBDII manuals, and repair guides.
Must Read: Check engine light flashing and car shaking
Table of Contents
What Is Check Engine Light and How to Diagnose?
The check engine light is a warning indicator on your dashboard that lights up when the onboard diagnostic system of the vehicle is triggered by the malfunctioning of different components of the engine.
A check engine light is usually illuminated if your car’s computer detects a malfunction in your engine. Check engine light (CEL) is also called Malfunction Indicator Light (MIL) and Service Engine Soon Light (SES).
If the check engine light comes on, the engine computer stores a “trouble code” that tells the source of the problem.
These trouble codes are identified by letters and numbers. Each letter represents a different type of malfunction. For instance, P in the trouble code of the engine means the fault is associated with the powertrain.
You can diagnose the check engine light and find out what code is associated with the check engine light by using Onboard Diagnostic (OBDII) scanner and plugging it into the port under the dashboard. You can also get this Bluetooth OBD2 reader.
Now, if the 0 comes after P in the trouble code, it means that it is a generic OBD code that applies to all vehicles. You can read more about generic trouble codes in the engine here. If the 1 comes after P in the trouble code, it’s a manufacturer-specific code. In this case, you have to check the owner’s manual to find the source of that trouble code.
In the video below, you can see that the guy used the OBD2 scanning tool to find the trouble codes in the engine. The OBDII tool displayed the P042 code which indicated the fault in the evaporated emission system (EVAP).
Usually, when a check engine light comes on and it doesn’t change how it’s running, it means the OBDII system has picked up an emissions-related fault.
These types of faults will not hurt the vehicle in the short term. You’d want to get the codes looked at as soon as possible, then get the issue fixed.
If the engine changes how it is running, then there is something wrong beyond emissions. You’d want to get it taken care of as soon as possible or you run the risk of damaging your vehicle further.
Check Engine Light Blinking or Comes On and Off
If the check engine light comes on and off i.e. flashing or blinking, it means that serious damage to the engine has occurred that has caused misfiring engine cylinders.
In such a case, you should immediately stop your car and take it to the mechanic as driving your car at this point could be severe enough to cause damage to the catalytic converter.
Also, when combustion in one or more of the engine cylinders takes place at the wrong time, it could lead to bent rods, damaged valves, damaged pistons, and severely reduced fuel economy.
So, you will end up paying a hefty amount for the damage if you don’t stop the car after CEL starts flashing.
Note: The OBDII standard (Onboard Diagnostics 2) used by the computer should be the same for all cars after 1996. The codes may be different, but the flashing light usually means there is a misfire in the engine.
On a forum, one guy commented that when the check engine light of his car was flashing or coming on and off, the issue was that the ignition coil of the engine was damaged.
The ignition coil is an induction coil in the engine that transforms low battery volts into thousands of volts to create a spark in the spark plug.
Another person commented that the reason for the check engine flashing in his car was due to the vacuum leaks.
In the engine, a vacuum leak means that a certain amount of air is leaking between the airflow sensor and the intake engine manifold. You can refer to this article about vacuum leaks for more info.
Most Common Check Engine Light Causes
Check engine lights can come on for all sorts of emissions-related issues.
Here are some of the most common causes of illumination of the check engine light even if your car is running fine.
- Loose gas cap
- Faulty O2 sensor
- Bad mass airflow sensor
- Bad catalytic converter
- Engine overheating
- Engine misfire
- Malfunctioning EGR Valve
- Bad fuel pressure regulator
- Clogged air filter
- Transmission problems
1. Loose or Bad Gas Cap
A loose or bad gas cap is the most common cause of lighting up a check engine light on the dashboard. The function of a gas cap is to entirely seal the fuse system so that fuel vapors do not leak out of a fuel tank, and no contaminants from outside can enter the fuel system.
Usually, when you don’t put the gas cap on properly after filling the tank, too much air gets into the system. Also, a loose gas cap prevents the fuel system from properly pressurizing.
There is a fuel tank pressure sensor installed on the fuel tank. If the gas cap is loose or bad, it will detect the leak in the EVAP system, which will trigger the ECM, and light up the check engine light. You can read my guide on P1450 code to learn more.
How to fix?
When you tighten the gas cap and turn it clockwise, make sure that you hear three to four clicks. However, if the seal of the gas cap goes bad, the tightening of the gas cap will not solve the problem. You have to replace it.
Moreover, check if there’s any dirt or gunk around the gas cap or the fuel filler neck of your car where you screw on the gas cap.
If you spot any dirt, debris, or even rust in those areas, it’s important to give them a good cleaning before you put the gas cap back on.
Also, take a look at the seal or o-ring on the gas cap to make sure it’s not cracked. Remember to lightly spray a little WD-40 on the o-ring seal of the gas cap, but be careful not to get any on the rest of the cap.
While you’re at it, check for any rust around the filler neck. If you find any, lightly sand it off using sandpaper or a steel brush. Don’t forget to place a rag in the gas filler neck to prevent debris from entering the tank.
Once that’s done, apply a small amount of silicone grease to the ring on the filler neck. This will help the fuel cap’s o-ring seal properly to the filler neck, preventing any leaks.
Note: If you need to replace a gas cap, avoid using aftermarket gas caps, as they may not fit correctly and can cause additional problems. It’s recommended to purchase a gas cap directly from the dealership or an authorized retailer for your specific vehicle make and model.
Also Read: Car won’t start after getting gas
2. Malfunctioning O2 Sensor
A problem with the oxygen sensor usually means that the car’s emissions system isn’t working correctly. The oxygen sensor is part of the car’s emission control system.
Oxygen sensors are used to detect the level of oxygen in the exhaust. When the sensor detects a low level of oxygen, it sends a signal to the car’s computer. The computer then issues a warning and turns on the CEL.
Oxygen sensors work by measuring the amount of oxygen in the exhaust and comparing it to a reference value. They can also monitor the air/fuel ratio of the mixture by sending real-time data of unburnt oxygen in the exhaust gases to the ECM.
A bad oxygen sensor not only lights up the CEL but also affects your engine performance. Due to improper air/fuel mixture, there will be a loss of engine power, and the fuel economy will also be affected.
How to diagnose?
In a vehicle, there are two oxygen sensors. One is before the catalytic converter and the other is located after it.
You have to diagnose the O2 sensor located before the catalytic converter. Pre-catalytic O2 sensor should show a varying voltage graph if it’s working fine. A bad O2 sensor shows the steady voltage.
How to test?
To diagnose the O2 sensor, you need an OBDII scanner. When you view the results in graph format, you’ll notice the voltage fluctuating between 0 and 0.5 volts while the engine is idle.
A voltage of 0 volts indicates a lean mixture, while 0.5 volts suggests a rich mixture. Now, when you rev the engine above 2000 rpm, it enters a closed loop and displays a voltage that corresponds to the oxygen content in the exhaust. Ideally, you want to see the voltage alternating above and below 0.5 volts.
Here’s how you interpret the results: If the voltage readings are consistently below 0.2 volts or above 0.7 volts, and they change rapidly, then congratulations! Your O2 sensor is working just fine.
However, if the voltage remains steady above 0.45 volts or below 0.45 volts, we need to dig deeper.
In such cases, it’s possible that your engine isn’t sufficiently heated up, as O2 sensors may give false readings when the engine is still cold.
To test this, rev the engine again above 2000 rpm. Now, let’s simulate a lean condition by opening the vacuum line.
Look for the power brake vacuum supply hose and use it for this purpose. By introducing a vacuum leak and observing its impact on the O2 sensor voltage, you can determine the sensor’s condition.
If the added vacuum causes the voltage to drop to 0.2 or 0.3 volts, or even lower, and you’re able to manipulate it by controlling the vacuum leak, then chances are your O2 sensor is functioning properly.
However, if you can’t get the sensor voltage to change no matter what you do to create a lean mixture, it’s likely that the O2 sensor is damaged.
I have explained the method of diagnosing oxygen sensors in my guide on the P0171 code.
3. Malfunctioning Mass Airflow Sensor
Mass airflow sensor (MAF) is located between the air filter and intake manifold (throttle body). MAF is a part of the engine’s computerized fuel management system. It is found in all modern gasoline engines and most diesel engines.
The mass airflow sensor (MAF) is an electronic component used to measure the amount of air entering the engine.
It measures the mass flow rate of air entering the engine and compares this with the desired flow rate of air entering the engine, thereby allowing the ECU to adjust the amount of fuel delivered to the engine.
The MAF (Mass Air Flow) sensor operates in an interesting way. It employs a heated wire or film, which gets cooled down by the incoming air, due to which its electrical resistance changes.
The change in electrical resistance of this wire or film is then used by the ECM (Engine Control Module) to calculate the precise amount of air that enters the engine.
Unfortunately, if the MAF sensor malfunctions or gets dirty, it can send inaccurate readings to the ECM, disrupting the engine’s performance and leading to unpleasant sputtering.
Note: Some engines have a Mass Air Pressure (MAP) sensor instead of a MAF sensor. So, it all depends on your engine. If the MAF sensor is not present, you should check the MAP sensor when your check engine light is on but the car is running fine.
How to diagnose?
If the mass airflow sensor is damaged, it will throw the reports out of range of what is expected which will cause the CEL to turn on. Codes are generally P0100-p0104, along with manufacturer-specific codes.
Before replacing the mass airflow sensor, you should also check vacuum leaks in the system as the air leakage through the vacuum hose and air intake manifold gasket can send wrong reports to the ECU, which can trigger the check engine light.
You can check out the below video to get an idea about the vacuum leaks.
To test MAF sensor, you need a multimeter. When the air flows through the MAF sensor at a higher rate, the voltage output also goes up. So, when you step on the gas pedal, more air passes through the sensor, causing the voltage to rise.
When the vehicle is idling, the voltage reading should be lower than 1.0V. However, as you increase the engine RPM, the MAF sensor produces a voltage output that goes from 1.0V to 1.7V.
If the MAF sensor is dirty or covered in debris and its voltage is fluctuating, it may be possible to clean the heated element of the sensor using a specialized cleaning spray.
I would recommend this cleaner. If your vehicle has a MAP sensor, it can’t be cleaned. You should replace it.
Here is a short YouTube video explaining how to clean an MAF sensor:
4. Bad Catalytic Converter
A catalytic converter is used to convert toxic gases into non-toxic gases. Catalytic converters are installed in gasoline engines to remove unburned hydrocarbons (HC) and carbon monoxide (CO) from the exhaust gases.
The exhaust gases from the engine contain these pollutants. The presence of these gases in the atmosphere is harmful to human beings. The main function of the catalyst is to reduce the level of these gases in the air.
A catalytic converter is damaged when raw unburnt fuel contaminates and clogs honeycomb-patterned substrate with oil deposits, soot, and water. This happens when unburnt fuel is accumulating in the catalytic converter due to an engine misfire.
When the catalytic converter becomes clogged, it restricts the flow of exhaust gases, leading to increased backpressure in the exhaust system. The added backpressure hampers the engine’s effectiveness in expelling exhaust gases with efficiency, leading to a decrease in power output.
Furthermore, catalytic converter uses oxygen in the exhaust gases to convert carbon monoxide to carbon dioxide and also to oxidize unburnt hydrocarbons. A bad catalytic converter will not be able to use the oxygen in the exhaust gases efficiently.
As a result, the oxygen sensor on the rear of the catalytic converter will record too much oxygen level in the exhaust gases. The sensor will send the data to the engine control module, which will turn on the check engine light.
5. Engine Overheating
Sometimes, overheating can also cause the check engine light to turn on. Overheating of the engine can lead to mechanical failure as the different components of the engine i.e. piston, cam phaser, cams, crankshaft, and connecting are subjected to very high temperatures that can affect the combustion. Resultantly, it can produce bad emissions, which can turn on the check engine light.
Overheating of the engine can result due to following reasons:
- Water pump has failed
- Oil has become bad
- Coolant level is low
- Blown head gasket
You can read my guide on P0128 code to understand the cooling system of the engine.
6. Fouled Spark Plugs
Spark plugs play a crucial role in how your vehicle’s engine performs. They are responsible for igniting the air-fuel mixture in the engine’s combustion chamber, providing the power that moves your car forward.
When one or more of your spark plugs become fouled, it can cause your check engine light to illuminate. Basically, fouled spark plugs cause misfires, due to which they can cause misfire codes like P0300, P0301, etc. The general format of the misfire code is P030X. ‘X’ is the cylinder number in which a misfire is occurring.
Two common types of fouling are oil-fouled spark plugs and carbon-fouled spark plugs.
- Oil-Fouled Spark Plugs: Oil leaks into the combustion chamber, coating the spark plug and preventing it from firing correctly. This insulating layer disrupts the spark plug’s operation. Causes for oil-fouled spark plugs can vary, including worn piston rings, valve stem seals, or a faulty gasket.
- Carbon-Fouled Spark Plugs: Carbon deposits accumulate on the spark plug’s electrodes, obstructing its ability to produce a spark. Several factors contribute to carbon fouling, such as low-quality fuel, frequent short trips, and stop-and-go traffic. Operating the engine at the wrong temperature can also lead to incomplete combustion, resulting in carbon fouling.
- Coolant-fouled spark plugs: If your engine is leaking coolant into the combustion chamber, it can coat the spark plugs and prevent them from firing correctly. This problem can be caused by a faulty head gasket, cracked cylinder head, or similar issues.
Aside from the electrode tip, the ceramic insulator of a spark plug can also become fouled. This ceramic insulator acts as a protective barrier, preventing electrical leakage that could damage the engine’s electrical system.
When the ceramic insulator becomes fouled with oil or contaminants, it causes an electrical leak across the insulator. This leads to incomplete combustion of the fuel mixture, resulting in misfires. In turn, this prevents the spark plug from generating the required spark, leading to issues like hesitation, reduced power, and acceleration trouble.
How to inspect?
To determine if your spark plugs are fouled, you can follow these steps:
- Use a spark plug socket to remove the spark plug.
- Visually inspect the spark plug for signs of fouling.
- Refer to a spark plug trouble tracer chart (PDF download available) for further guidance.
How to clean them?
The decision to clean or replace spark plugs depends on their condition. If the spark plugs are worn out or damaged, replacement is necessary. However, if they are still in good condition, cleaning and re-gapping can be performed.
The cleaning process for fouled spark plugs involves the following steps:
- Use compressed air to remove dirt and debris from the spark plugs.
- Soak the spark plugs in brake cleaner to wash away any remaining residue.
- Use a nylon brush to effectively loosen and remove debris.
By understanding the causes of fouled spark plugs and following proper cleaning or replacement procedures, you can address the check engine light issue and maintain optimal performance for your car.
7. Bad Ignition Coil
An ignition coil is a clever device that plays a crucial role in your vehicle’s engine performance. It converts the battery’s low voltage into a high voltage, producing the spark needed to ignite the fuel-air mixture in the spark plugs. There are two primary types of ignition coils: coil over plug (COP) and coil pack with distributor. Let’s dive into each of them.
Coil Over Plug Ignition Systems (COP)
Also known as direct ignition systems, COP ignition systems utilize individual ignition coils for each spark plug. Unlike traditional ignition systems that rely on a distributor to distribute the high voltage to all spark plugs, COP systems eliminate the need for a distributor. This simplification ensures that each spark plug consistently receives the necessary voltage promptly.
Distributor-Based Ignition Systems
Contrarily, distributor-based ignition systems use a single ignition coil to generate high voltage. The high voltage is then delivered to each spark plug through a distributor cap and rotor mechanism. The distributor cap and rotor work harmoniously to ensure that the correct spark plug receives the high voltage at the precise moment.
To visualize the process, the distributor’s shaft rotates as the camshaft turns. The rotor, connected to the shaft, spins inside the distributor cap. This cap houses a spring-loaded carbon brush that stays in contact with the rotor’s metal part.
As the rotor’s outer edge passes each internal plug terminal in the distributor cap, the spark plugs fire in the accurate sequence. This coordinated firing ensures efficient engine operation.
Signs of a Faulty Ignition Coil and Effects
A faulty ignition coil can cause various issues, including a flashing check engine light. When an ignition coil fails, it disrupts the engine’s firing order, leading to misfires. Misfires occur when the fuel-air mixture fails to ignite correctly in one or more cylinders. As a consequence, the engine may run rough, idle poorly, or even stall.
When the engine’s computer detects these misfires, it promptly triggers the check engine light, which will begin to flash. This distinct warning flag indicates an immediate need to address the engine problem.
Ignoring this issue could result in severe damage to the engine and other vehicle components due to excessive vibrations.
Coil Pack Ignition System
An alternative to COP ignition systems is the coil pack ignition system. It utilizes a single coil to fire multiple cylinders. The coil pack is mounted on the engine and connected to the spark plugs through individual spark plug wires.
How to troubleshoot?
Now, let’s explore how you can determine if your ignition coil is faulty.
- For COP ignition systems, one effective method is to swap the ignition coil from the problematic cylinder with an ignition coil from a different cylinder. Then, observe if the trouble code changes. If it does, it indicates a faulty ignition coil.
- Another way to check a bad ignition coil is that if you remove the ignition coil of a certain cylinder and the engine starts to stumble, it means the ignition coil is good. If the engine stays the same, it means the ignition coil is bad.
Troubleshooting Distributor-Based Ignition Systems
In distributor-based systems, it is essential to keep an eye on the distributor cap and rotor.
Over time, the contacts inside the distributor cap can corrode or wear down, weakening the spark. Similarly, the rotor itself can wear down, resulting in poor contact with the distributor cap’s contacts.
How to troubleshoot bad ignition coil pack?
For ignition coil packs, you can watch the following video:
8. Vacuum Leaks
Vacuum leaks occur when air enters the engine through an unintended path, rather than the intended throttle body.
This influx of air, known as “unmetered air,” is not measured by the Mass Air Flow (MAF) sensor or the Manifold Absolute Pressure (MAP) sensor.
As a result, the air-to-fuel ratio becomes imbalanced, leading to various issues like sputtering during acceleration at low RPMs.
The amount of air entering the engine through a vacuum leak can vary depending on the size of the leak and the position of the throttle.
While the engine is operating, a vacuum is created, drawing in air, fuel, and exhaust gases. This vacuum is produced by the downward motion of the pistons during the intake stroke.
During idle, the mostly closed throttle plate limits the air intake, generating a high-pressure area in the intake manifold and a low-pressure area in the engine’s combustion chambers. This pressure differential creates the vacuum effect.
However, pressing the accelerator pedal opens the throttle plate, allowing more air to enter the engine. This raises the pressure in the intake manifold and reduces the vacuum effect.
As there is greater vacuum in the engine at low RPM, any vacuum leak results in a significant amount of “unmetered air” entering and disrupting the air-fuel ratio. When the ECU detects any disruption in the air-fuel ratio, it will turn on the check engine light. The most common OBDII code indicating vacuum leaks is P0171.
how to find vacuum leaks?
- Damaged or Cracked Hoses: The primary cause of vacuum leaks is often damaged or cracked rubber hoses. Over time, the engine’s air intake hose can become brittle and develop cracks. These cracks create openings in the intake system, enabling unplanned air entry into the engine. This negatively affects engine performance. To address this issue, inspect the air intake hose connecting the throttle body to the air intake manifold.
- Worn or Damaged Intake Manifold Gasket: The intake manifold gasket seals the intake manifold to the engine block. Over time, wear and damage can occur, allowing air to enter the engine from sources other than the air filter.
- Cracked or Damaged Vacuum Hoses: Vacuum hoses transport air to and from various components, including the EVAP purge valve, fuel pressure regulator, and PCV valve. If these hoses develop cracks or sustain damage, they can permit air leakage into the system, resulting in operational problems.
- Throttle Body Gasket Failure: The throttle body gasket seals the throttle body to the intake manifold. If this gasket fails, a vacuum leak can occur. The vacuum pressure draws air into the engine through the gasket gap, negatively impacting engine performance.
- Loose or Damaged Fuel Injector O-Rings: Fuel injectors deliver fuel into the engine and are sealed using O-rings. If these O-rings become loose or damaged, they can allow air to enter the engine, creating a vacuum leak.
When a vacuum leak is present, it is often accompanied by a hissing sound from the engine. You can use carb cleaner to detect vacuum leaks in the engine.
You can watch the below youtube video to diagnose vacuum leaks:
9. Malfunctioning EGR Valve
EGR stands for Exhaust Gas Recirculation. The EGR valve is used to reduce emissions by recirculating exhaust gas back into the combustion chamber.
EGR helps reduce the combustion temperatures and the number of NOx gases in the engine exhaust. The temperature of combustion is reduced because less oxygen is needed from the air for combustion.
A frequent problem with the EGR valve is that it is often stuck open or stuck close due to a build-up of carbon deposits. If EGR valve is stuck open, it causes an imbalance in the air-to-fuel ratio, leading to engine misfires and reduced power. In such cases, the vehicle’s onboard diagnostic system detects the irregularity and activates the check engine light to alert you of the problem.
If EGR valve is stuck close due to carbon deposits, it will result in increased emissions.
When the valve fails to recirculate the correct amount of exhaust gases, the engine may produce higher levels of nitrogen oxides (NOx) and other pollutants. This can not only harm the environment but also cause the vehicle to fail emission tests.
You can safely drive with a clogged EGR valve. But, if you do not clear or replace the EGR valve, it can cause head gasket failure.
How to test?
Remove the EGR valve and perform a visual inspection to check for any cracks or holes in the housing. A cracked or damaged EGR valve will need to be replaced.
To check if the EGR valve is working correctly, we will now use a hand-operated vacuum pump.
If the EGR valve can hold a vacuum, it indicates that the valve is functioning properly. However, if the vacuum pressure drops off after a while, it suggests a faulty EGR valve that needs to be replaced.
10. Bad Fuel Pressure Regulator
A bad fuel pressure regulator can also cause illumination of the check engine light. The purpose of a fuel pressure regulator is to maintain fuel pressure inside the fuel rail so that fuel injectors inject the fuel at a certain pressure.
Keep in mind that Gasoline Direct Injection (GDI) engines have a fuel pressure sensor instead of a regulator.
So, if you have GDI engines, you should diagnose the fuel pressure sensor. You can read my guide on bad fuel pressure regulator symptoms to understand how fuel pressure regulator works and how it becomes bad.
11. Clogged Air Filter
We usually forget to replace the engine air filters. An air filter is an important part of an engine. It catches dust, dirt and debris before they enter the intake manifold. It also helps keep harmful particles out of the engine.
When the air filter gets clogged, the engine can’t get sufficient air for efficient combustion. In that case, your car will still run fine but the ECU will turn on check engine light. You should replace the engine air filter every 20,000 to 30,000 miles.
12. Transmission Problems
An automatic transmission system engages and disengages gears by supplying current to the solenoid in the valve body of the transmission to allow transmission fluid to flow and clutches to engage.
The vehicles also have a transmission speed sensor that determines the speed of gears in the transmission and sends signals to the ECU.
Similarly, the transmission Fluid Temperature sensor measures the temperature of the transmission oil and sends signals to the ECU.
When the ECU detects any problem in transmission oil or components of transmission system, it will turn on check engine light.
If there are leaks of transmission fluid, you should fix them. To learn more, you can read my guide on how much it costs to fix transmission leaks.
So, if the check engine light is on but the car is running fine, the problem is not too serious. You need to check the gas cap, engine cooling system, catalytic converter, and engine sensors.
Make sure that you also check the harness connectors of engine sensors. They should supply a proper 5V or 12V to the engine sensors.
The wires in the connectors should be tight. When you tug on each wire in the connector, it should not come loose.
If the check engine light is blinking, the problem is serious as it indicates an engine misfire. In that case, you should check spark plugs, ignition coils, fuel delivery system, catalytic converter, and vacuum leaks.
Sometimes, low oil pressure can also turn on the check engine light. So, make sure to check it as well.