Check Engine Light On But Car Runs Fine (10 Common Causes)

A check engine light is one of the most annoying things that can happen to your car. The check engine light is a tool to assist with diagnostics and maintenance of your vehicle. When the check engine light comes on, there is an issue with the car and you need to take it to a mechanic. The issue with the car can be minor like a loose gas cap, or clogged air filter, and major like a clogged catalytic converter or bad oxygen sensors. In this blog, I’ll explain some important causes from my experience that can cause a check engine light to turn on while driving despite a smooth engine operation.

The most common causes of check engine light on but car runs fine are the clogged air filter, bad MAF sensor and bad oxygen sensors. Check engine light always illuminates when something is wrong with the engine. It could be a simple or complex issue in the engine. If a car runs fine when check engine light is on, you should look at the emission system of the engine. Another reason for illuminating the check engine light is the loose or bad gas cap. You should always get the code using an OBDII scanner to instantly find the main reason that is turning on the check engine light. Also, note that if the check engine light is blinking or flashing, an engine misfire is occurring which can cause serious damage to your engine. So, if instead of a static CEL, you’re seeing a flashing CEL, you should immediately stop the car and address the issue. 

There are dozens of different symptoms that can point to a car’s check engine light coming on, and while they don’t all cause a problem with vehicle performance straight after your check engine light comes on, at least you know what the common issues are that could develop later and get worse over time. Sometimes, this light will come on due to a sensor fault inside your car. Sensors keep an eye on almost everything from your oil pressure to your fuel gauge, so it’s no surprise if every once in a while one of these fails. Here we’ll take you through the most common causes due to which my check engine light comes on, and what you can do about them.

Check Engine Light Orange or Yellow and Is SteadyCheck Engine Light Red and Blinking/Flashing
Bad or loose gas capEngine misfire
Bad EGR valveBad spark plugs and ignition coil
Leaks in EVAP systemClogged fuel injector
Malfunctioning O2 and MAF sensorVacuum leaks after MAF sensor

Note: The information presented in this article is gathered from online automotive forums, OBDII manuals, and repair guides.

What Is Check Engine Light in Cars?

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 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

causes of check engine light on but car runs fine

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.

  1. Loose gas cap
  2. Faulty O2 sensor
  3. Bad mass airflow sensor
  4. Bad catalytic converter
  5. Engine overheating
  6. Engine misfire
  7. Malfunctioning EGR Valve
  8. Bad fuel pressure regulator
  9. Clogged air filter
  10. Transmission problems

1. Loose/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.

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.

Your car won’t start after getting the gas? Check out this guide.

2. Faulty 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.

Diagnosing the oxygen sensor in the engine requires a digital multimeter. Each car model has a different number of oxygen sensors, installed in different locations. So, to diagnose the oxygen sensor, you might need to take your car to the mechanic.

I have explained the method of diagnosing oxygen sensors in my guide on the P0171 code.

3. Bad 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 a device 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.

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.

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.

4. Bad Catalytic Converter

catalytic converter schematic

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.

A 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

You can read my guide on P0128 code to understand the cooling system of the engine.

6. Engine Misfire

Engine misfire is a more serious cause of the check engine light compared to the above causes. It usually blinks or flashes the check engine light. Engine misfire occurs when there is incomplete combustion inside engine cylinders. Engine misfire can occur due to following reasons:

  • Bad ignition coil
  • Worn spark plug and wires
  • Vacuum leaks
  • Malfunctioning fuel injector

The most common cause of misfires is a faulty ignition system or a bad spark plug. In gasoline engines, the combustion of fuel entirely depends on the spark plug and its wires. The wires of the spark plug drive the current from the ignition coil. When either of these components (ignition coil, spark plug, and wires) is damaged, there will be an engine misfire.

Note: Remember that the configuration of the ignition coil depends on the engine. Some engines have distributors while some have ignition coils plugged into the spark plug. In my guide on check engine light flashing and car shaking, I have explained different configurations of ignition coil and methods to test a bad ignition coil.

A bad cam phaser can also be the cause of engine misfire. Check out my guide on a bad cam phaser.

A dirty or malfunctioning fuel injector can also be the cause of lighting up a check engine light on the dashboard. The faulty injector will not deliver the fuel to the engine at the proper time, resulting in poor combustion. The defective injector will also cause the engine to run lean, which can result in poor fuel economy. If less fuel is injected into the cylinder, the engine control unit will detect this fluctuation and will trigger the check engine light.

In my guide on car won’t start after replacing fuel injector, I have discussed a method of testing fuel injectors.

Vacuum leaks in the engine can also cause the engine to misfire and shaking of your car. During the intake stroke, a vacuum is created in the engine that sucks the air into the cylinder. If there is any leakage in the intake manifold after the MAF sensor, outside air will enter the engine, which will not be detected by the MAF sensor. As a result, ECU will not be able to recognize that air. The mixture will run lean and a misfire will occur. You can use carb cleaner to detect vacuum leaks in the engine.

You can watch the below youtube video to diagnose vacuum leaks:

7. 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. It 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 due to a build-up of carbon deposits. The EGR valve and the exhaust gas recirculation passages are blocked by the carbon deposits. Blocking the EGR valve can increase fuel consumption and reduce engine performance, which can turn on the check engine light.

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.

8. 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.

9. 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.

10. 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 oil to flow and clutches to engage.

valve body of transmission system with solenoids

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.

Final Thoughts

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.