The check engine light, or CEL, usually comes on and goes off when you turn the ignition on. If the check engine light came on and then went off while the engine was running, it indicates the following problems.
A loose or faulty gas cap is the most common reason for this issue. Spark plug issues, faulty oxygen sensors, or stuck EGR valves can also cause the check engine light to come on and then go off. Other reasons include faulty mass airflow sensors and catalytic converters.
Even though this problem isn’t something to panic about, prompt resolution of the problem is necessary. I will explain the detailed steps of solving the problem. Let’s begin.

Check Engine Light Came On Then Went Off: Why Does It Happen?
Let me explain the problems that can cause the check engine light to come on and go off. You will also find the detailed solutions below.
1. Loose or Faulty Gas Cap
Each time you fill the gas in the tank, you should tighten the gas cap properly. If the gas cap is loose, fuel vapor can escape the tank. The computer in your car will consider this an evaporative system leak and turn the check engine light on.
Be careful when filling gas in your car. Overfilling the gas tank can cause the check engine light to come on. When the excess gas is used or you tighten the gas cap next time, the check engine light will go off.
How to Fix It?
- Park your car somewhere safe and turn it off.
- Check if the gas cap has any play. If so, probably the loose gas cap is the reason behind the problem.
- Open the cap and check the rubber O-ring inside it. If the ring is worn or cracked, replace the O-ring.
- Then, tighten the gas cap properly. You should hear a clicking noise, which indicates the cap is tightened properly.
2. Faulty Spark Plugs
Over time, carbon deposit on spark plugs can reduce their efficiency. As a result, the plugs won’t make sparks every time you fire the engine. The system will detect it as an engine problem and throw the check engine light.
When the spark plugs work correctly the next time, the check engine light will automatically go off. But you shouldn’t ignore the issue just because the light went off.
How to Fix It?
- Turn the car off and let it cool down.
- Open the hood and disconnect the spark plug wires.
- Remove the spark plugs and inspect them for carbon deposits.
- Clean the spark plugs using a spark plug cleaner. I suggest using the Pneumatic Spark Plug Cleaner for the best result.
- Assemble the spark plugs and wires after cleaning. The engine should start easily, and the check engine light won’t turn on again.
3. Bad Oxygen Sensors
The air-fuel mixture inside the combustion chamber needs to be burned properly for the engine to be the most efficient. It can be determined by measuring the amount of oxygen in the exhaust gas. Oxygen sensors are responsible for this.
If the sensor doesn’t work properly once or twice, it won’t measure the amount correctly. The ECU will consider it as an engine problem and trigger the CEL. When the sensor starts working, the check engine light will go off.
How to Fix It?
- Use an OBD2 scanner to read the error code. In most cars, the error code should be anything between P0171 and P0175. These codes indicate the problem is with the oxygen sensor. It may seem a bit confusing as the check engine light is already off. For your information, a car can throw codes without the check engine light.
- Locate the oxygen sensor. It should be close to the catalytic converter in the exhaust system.
- Remove the electrical connector from the sensor. Then, use a flathead screwdriver to pry out the sensor.
- Install a new sensor and press the tab firmly. Then, attach the electrical connector.
4. Stuck EGR Valves
EGR valves help recirculate some of the exhaust gas into the engine’s air intake. It increases the efficiency of the engine. If the EGR valve is stuck in a partially closed position due to clogs or damage, the ECU will detect a low-efficiency issue. So, the check engine light will turn on.
If the EGR valve is stuck temporarily due to soot, the error will also be temporary. When the valve moves freely, the check engine light will go off.
How to Fix It?
- You need to remove the EGR valve from the housing. It is near the intake manifold of the car.
- Use a wrench to loosen the screws and take the valve off. Also, check the valve gasket and replace it if the gasket is worn.
- Install a new EGR valve. Make sure the gasket and the valve are aligned and tightened properly.
- If you have replaced the EGR valve, but the light is still on, you should check the condition of the new valve and reinstall it.
5. Faulty Mass Airflow Sensors
This sensor is responsible for allowing the right amount of air into the engine. If the sensor is dirty or malfunctioning, the air-fuel mixture can be rich or lean. In that case, the ECU will trigger the check engine light. The problem can be intermittent, so the light may sometimes go off automatically.
How to Fix It?
- You need to remove the mass airflow sensor from the housing.
- Clean the dirt and grime from the sensor and install it.
- If the check engine light is still on, you need to replace the sensor.
- Take the old sensor off and install a new sensor.
6. Bad Catalytic Converter
Modern vehicles have catalytic converters to reduce harmful emissions. If the converter doesn’t work at the threshold efficiency or higher, the ECU will trigger the check engine light. This can be a temporary situation, caused by low-quality fuel.
When the catalytic converter crosses the threshold, the check engine light will go off.
How to Fix It?
- Use a diagnostic scanner to read the error code when the check engine light is on. If the light goes off, still get the catalytic converter cleaned.
- You should contact an expert mechanic to get the catalytic converter repaired. Don’t work on it yourself unless you have extensive experience with it.
Conclusion
Check engine light coming on and going off, which indicates minor problems with the car. But you shouldn’t ignore the problem because the underlying reasons can extend to expensive repairs. Understanding why the check engine light came on and then went off and knowing the fixes is essential.
I mentioned six reasons behind the problem and included step-by-step solutions for each reason. Even if the check engine light has gone off, you should still diagnose the proper reason and implement the solution. It will ensure the problem doesn’t come back. Keep an eye on small issues, such as tightening the gas cap, cleaning the spark plugs, etc., to avoid the problem frequently.