If check engine light illuminates on the dashboard and is not red or flashing, you can drive the car safely as long as you have already checked the oil level and its pressure, serpentine belt, coolant, transmission fluid and battery, and your engine is not overheating. If the check engine light comes on steadily and there are no overheating, engine misfire or low oil pressure issues in your engine, you can drive your car up to 100 miles to visit an auto repair shop. Check engine light (CEL) could be on due to several reasons. It would be best to always take the OBD2 scanner with you and get the codes that are causing CEL to turn on.

You can also read my guide on check engine light on but car is running fine to get an idea of some most common causes of check engine light.

What Is Check Engine Light?

Check engine light (CEL) is a warning light that appears on the dashboard of a vehicle when the engine control system detects any problem with the vehicle’s emissions system. Check engine is also called ‘Malfunctioning Indicator Light (MIL)’ in some engines. Sometimes, CEL is just a warning sign; however, it can also be a sign of a bigger problem. A problem with the engine can be something that is a quick and cheap fix or it could be a problem that will take several days to fix and hundreds of dollars.

A check engine light could indicate that your vehicle needs a tune-up or other maintenance work. A mechanic can diagnose the problem and fix it. A mechanic can also test the health of the engine by checking the oil level and the coolant level.

The ECU is programmed to work with different engine sensors (O2 sensor, mass airflow sensor, etc) to set the required air/fuel ratio for efficient engine performance and to control emissions. If the ECU detects the difference between the values it receives from engine sensors and the values on which it is programmed by the manufacturer, it will illuminate the check engine light.

In engines, a display message also appears on the dashboard when the check engine light is on. This display message depends on the engine. 

For instance, in Toyota RAV4, the display message of the check engine light is:

“A malfunction in electronic control of the engine, throttle, or automatic transmission has been detected. Contact your dealer to have your vehicle inspected immediately.” 

In some engines, the check engine light appears as a “Service Engine Soon” message.

Check Engine Light is mainly triggered when there is a problem in the vehicle’s emission system. But other issues, such as malfunctioning transmission system, overheating, spark plug damage, fuel pressure, low oil pressure, ABS and traction/stability system can also turn on the check engine light along with other warning lights on the dashboard in modern engines. 

With the check engine light, your vehicle can also put be put into reduced engine power mode by the ECU to prevent further engine damage and allow you to drive the vehicle safely to your desired place.

You should always have an OBD2 scan tool on hand so you can check your OBD2 code and see if your engine has a critical problem or if you can drive safely for some distance to reach your destination. 

My favorite OBD2 scan tool is Bluedriver. It is quite handy and affordable. You can connect it to your smartphone via Bluetooth.

What Does Yellow Check Engine Light Mean?

Check engine light can either be red or yellow/orange. Yellow or orange check engine light does not mean you stop the vehicle immediately. Yellow check engine light means driving the vehicle to a nearby auto repair shop and fixing the minor issue as soon as you can. With a yellow check engine light, the engine most probably has a minor issue and is not dangerous for your vehicle. However, this minor issue could cost you in the long run if you don’t take precautionary measures at the right time.

When the check engine light is on and is steady, just make sure that you’re not aggressively accelerating your vehicle and you should keep the speed of your vehicle under 50 miles/hour. If you’re driving on a highway, you should drive at a constant speed. Do not accelerate or brake too hard and suddenly. Moreover, take the turns smoothly while you’re driving your car with check engine light on.

If the check engine light is red and flashing, you should immediately stop your engine and don’t drive anywhere. You can read my guide on check engine light flashing and car shaking to learn about the most common causes of a check engine light blinking.

What Is The First Thing To Check When The Check Engine Light Comes On?

The first thing you need to check when the check engine light comes on is the level of motor oil, coolant, and transmission fluid if you have an automatic transmission system. You should check the engine air filter as well.

If the gauge on the dashboard is showing low oil pressure or an engine overheating warning, you should immediately check the oil condition and the causes of engine overheating. You can read my guide on P0128 code to get an idea of how the engine’s cooling system works.

The next thing you should check when the check engine light comes on is the condition of the serpentine belt. The serpentine belt runs auxiliary components of engines like alternator, water pump, etc. 

Also, you should check your engine’s ground connections and harness connectors of engine wiring that supply voltage to the engine’s sensors if the check engine light comes on. You can read my guide on the car won’t start after replacing battery to learn how to diagnose bad ground connections.

Common Causes Of Check Engine Light

Here are some of the most common causes of a check engine light:

• Loose gas cap
• Bad air filter
• Bad oxygen sensor
• Bad mass airflow sensor
• Vacuum leaks
• Bad fuel tank pressure sensor
• Leaks in the EVAP system
• Bad fuel pressure regulator/sensor
• Bad catalytic converter
• Bad EGR valve
• Bad spark plugs

1. Loose Gas Cap

Loose gas caps are one of the most common reasons why your vehicle’s check engine light turns on. Some vehicles like Ford Escape also separately show the message “Check Fuel Cap” when the check engine light turns on. A gas cap is a part of the EVAP system.

A loose or bad gas cap allows air to mix with the fuel vapors and affects the desired vacuum that should be present in the EVAP system or cause fuel vapors to escape from a gas tank, reducing the fuel mileage and harming the environment.

A loose gas cap is caused for several reasons. One of them is the cap can’t be tightened properly. Another one is the cap is old and weak. You also need to clean the cap regularly. The dirt and moisture may cause the gas cap to fail. Another hint of a bad gas cap is that it has a stick-slip feeling when tightened.

The issue of a loose gas cap is not detrimental to your engine and you can drive your vehicle as long as you want if the check engine light is caused by a loose gas cap. however, since a loose gas cap can cause an excessive release of fuel vapors into the atmosphere, it would be good if you fix the gas cap. 

Moreover, if there is an excessive vacuum in the fuel tank, a gas cap allows for relieving the excessive vacuum so that the fuel pressure pump can easily pump fuel from the fuel tank. So, whenever you get a chance, you should check the gas cap.

Make sure that the seal of the gas cap is not cracked. You should tighten the gas gap until you hear 3 or 4 clicks. Also, check if there are deposits of carbon or dirt around the seal of the gas cap or around the fuel filler neck of your vehicle in which a gas cap is tightened.

2. Malfunctioning Fuel Tank Pressure Sensor

A malfunctioning fuel tank pressure sensor can cause a check engine light to come on. A fuel pressure tank pressure sensor is also called an EVAP sensor or fuel vapor pressure sensor. 

The fuel tank pressure sensor is a small device that sits in or is placed on the top of the fuel tank. It monitors the amount of pressure that is being built up in the tank. When the fuel vapor pressure gets too high or too low, the sensor triggers a warning light to let you know that there is a problem. For a bad EVAP sensor, you might get P0440 or P0450 OBDII codes.

EVAP sensors can malfunction because of corrosion, which weakens the electrical connection, or a damaged electrical connector. So, first, make sure that you’re getting 5V on the voltage signal terminal of the fuel tank pressure sensor. For this, you first have to verify the signal wire in the harness connector.

Turn the ignition key on. Set multimeter to DC voltage. Connect the black probe of the multimeter to the engine ground and the red probe of the multimeter to each terminal of the harness connector. There would be one terminal that would give around 5V on the multimeter.

To test the FTP sensor, you need a vacuum pressure tester and a multimeter. Turn on the ignition key. Connect the black probe of the multimeter to the engine ground and the red probe of the multimeter to the signal wire of the FTP sensor. Now, if the voltage decreases when you apply the vacuum, it means the FTP sensor is fine.

3. Leaks In EVAP System

EVAP stands for ‘Evaporative Emissions Control System‘. It consists of a charcoal canister, an EVAP purge valve, and a canister valve. If any leak in the EVAP system is detected, it will also turn on the check engine light. EVAP system leaks are also not detrimental to your engine. But, to prevent fuel vapors leaks into the environment, you should fix this issue.

The purpose of the EVAP system is to capture extra fuel vapors from the fuel tank through the charcoal canister and re-route it to the engine air intake through the EVAP purge valve to burn with the air. EVAP purge valve only opens when ECU sends signals to it. The canister vent valve remains open all the time so that air can be sucked in to carry fuel vapors with it. The canister vent valve is only closed when a leak has to be tested in the EVAP system. 

So, you should check if the EVAP purge valve is stuck open or stuck closed. Moreover, you should also check if the canister vent valve is stuck open.

You can have a smog test for your EVAP system to detect leaks.

4. Bad Oxygen Sensor

A bad oxygen sensor is the most common reason for the illumination of the check engine light. In engines, there are two oxygen sensors. One is installed before the catalytic converter and a secondary O2 sensor is installed after the catalytic converter. 

The primary oxygen sensor checks the oxygen levels in the exhaust gas and sends signals to the ECU so that it can decide how much fuel should be injected. If the primary oxygen sensor becomes bad, it will cause the air/fuel mixture to become lean or rich. If the air-fuel mixture is either too rich or lean, inefficient combustion will take place. It will also cause the engine misfire, damage the catalytic converter, and also result in poor fuel economy.

The O2 sensor after the catalytic converter to determine whether the catalytic converter is working properly. If this O2 sensor reports a high oxygen level, the ECU will interpret a catalytic converter is bad and will turn on the check engine link.

O2 sensor can malfunction due to the following reasons:

• Soot deposits
• Water in the connector
• Sensor is bent
• Frayed or broken wires
• Wires melted by contact with the exhaust
• Wire grommet is loosened, which can cause water to enter the O2 sensor

5. Bad MAF Sensor

A malfunctioning MAF sensor also turns on the check engine light. Keep in mind that not all engines have MAF sensors. Some engines have MAP (Manifold Air Pressure Sensors). So, it depends on your vehicle.

MAF sensor is present after the engine air filter that sends signals to the ECU about how much air is entering the engine so that the ECU adjusts the fuel injection accordingly.

Before replacing the MAF sensor, you should first try cleaning it. 

To clean the MAF sensor, follow these steps:

• Disconnect the negative terminal of the battery.
  Remove the MAF sensor, it is located on the intake tube between the air filter box and the throttle body facing upwards.
• Spray all of the little sensors you see inside of the MAF carefully with this MAF sensor cleaner. Avoid touching the wires of the MAF sensor. It will damage it.
• Reinstall and reconnect the battery terminal.
• Start the engine and let the ECU relearn idle for 15 minutes.

To test the MAF sensor, you can watch the below Youtube video:

6. Vacuum Leaks

Vacuum leaks mean the introduction of unmetered air through vacuum lines, throttle body gaskets, Throttle position sensors, or air intake manifolds. When the vacuum system fails, it will cause the Check Engine Light to come on.

MAF sensor is mounted before the throttle body to measure the amount of air passing through it. If the air is introduced into the air intake system after the throttle body, the ECU couldn’t recognize that air and adjust the fuel amount. This will turn on the check engine light.

You can use carb cleaner to detect vacuum leaks in the engine.

7. Bad Fuel Pressure Regulator

The purpose of a fuel pressure regulator is to maintain a certain fuel pressure inside the fuel rail so that the fuel injector injects the fuel at a certain pressure and doesn’t damage the spark plugs and cause the engine misfire. If the fuel pressure regulator fails to maintain the fuel pressure, it will also turn on the check engine light. You can read my guide on symptoms of bad fuel pressure regulator to learn more.

If you have a Gasoline Direct Injection engine, it will have a fuel pressure sensor instead of a fuel pressure regulator. This fuel pressure sensor sends signals to the ECU to control the solenoid valve of a camshaft-driven GDI fuel pump.

8. Bad Catalytic Converter

A catalytic converter is designed to convert harmful exhaust emissions into harmless gases. However, a defective catalytic converter can cause problems in your engine. Catalytic converters tend to block up and become less effective over time. When this happens, you may notice a Check Engine light come on. The catalytic converter is activated by the engine’s exhaust gas temperature. If the catalytic converter is clogged, it will also cause overheating problems in the engine, and can also affect fuel economy. You can read my guide on CAT Delete to understand the operation of a catalytic converter.

9. Bad EGR Valve

EGR stands for Exhaust Gas Recirculation. EGR system is used by diesel engines to retract exhaust gases to the air intake manifold so that their heat energy can be utilized and efficient combustion can be taken place.

Due to exhaust gas recirculation, NOx emissions can also be reduced in the exhaust gases. Since a malfunctioning EGR valve affects your vehicle’s emission system, it will turn on the check engine light.

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.

You can safely drive your vehicle with the malfunctioning EGR valve as it doesn’t your engine. However, when you get a chance to visit an auto repair shop, you should replace the EGR valve. The error codes related to the EGR system will be P0400, P0401 or P0402.

10. Bad Spark Plugs

A bad spark plug is also responsible for Check Engine Light On. The spark plug delivers a spark when the ignition coil applies a high voltage current to it. As a result of the spark, the air/fuel mixture ignites to produce power.

A bad spark plug can result in the misfiring of the engine, which can cause a variety of problems. A bad spark plug can even cause the car to stall out. If the engine misfires, it will cause the vehicle to lose power and it will not produce the desired level of performance.

With time, a spark plug is fouled with the fuel deposits, ash deposits, corrosion and engine oil deposits. Usually, in owner’s manuals of vehicles, it is recommended to replace spark plugs every 30k to 60k miles. 

I have found this amazing guide (bad spark plug signs PDF) that includes pictures of all possible damages with the spark plug. 

How Long Does Check Engine Light Stay On After Fixing The Problem?

The check engine light stays on for up to 20 drive cycles after you fix the problem. After completing 10 to 20 drive cycles, the check engine light should come off if you have fixed the problem. Sometimes, an error code is stored in the memory of the engine’s computer. The best practice is to always clear the codes and reset the ECU using the OBD2 scan tool after you fix the problem.

Drive cycle is a term used in vehicle repair to indicate a series of vehicle trips. Usually, one drive cycle consists of a small trip that completes in around 20 to 30 minutes. After a repair has been completed, drive cycles allow the ECU to execute all the monitoring systems so that the ECU verifies if the problem still exists.

Will My Car Emission Test If I Reset The Check Engine Light?

You should not perform an emission test on your vehicle immediately after fixing the problem that is turning on the check engine light. Since the check engine light is mainly related to the defect in emission systems, you should complete certain drive cycles and see if the check engine light turns off or not after around 20 drive cycles. After completing the drive cycles and confirming that the check engine light remains off, the ECU will relearn the system and will enable your vehicle to pass the emission test.

Conclusion: How Long Can You Drive with the Check Engine Light On?

If the check engine light is on and steady, you can drive as long as your car runs smoothly. As a rule of thumb, you should not drive more than 100 miles when the check engine light is on. Use the OBD2 scan tool to get trouble codes and fix the problem. Don’t forget to clear codes from the engine’s memory. After fixing the problem, complete certain drive cycles and also pass the emission test.