How to troubleshoot- check engine light on but car runs fine? (5 Commons Causes)

It’s a frustrating experience when you see the check engine light on, but everything is running fine. So, most drivers ignore this, but it can lead to severe issues that could become a bigger problem down the road. It is essential to troubleshoot and fix the problem. So, Why “check engine light on but car runs fine

Start by checking the gas cap first, ensuring it is tight. If the problem exists, then check the oxygen sensor, catalytic converter, ignition coil, and spark plugs. Once you find the cause of the problem, fix it by following the instructions. Remember to reset the check engine light before departure.

Stay with me, I will explore tips and tricks for troubleshooting a check engine light that is on, but the car seems to be running smoothly. Let’s explore.

Understanding the Check Engine Light

check engine light on but car runs fine

The check engine light is installed in modern vehicles as a warning indicator. If the check engine light is illuminated, you have an engine or emission system issue. You have to check for all the possible causes and fix the issue. Otherwise, you may experience severe engine or emission system problems that may lead to serious damage to your vehicle.

Possible causes for check engine light on but car runs fine:

There can be several possible causes for a check engine light being on, but the vehicle is running fine. However, if your check engine light is illuminating, you must stop the vehicle and check for the reasons below.

  • Loose gas cap
  • Oxygen sensor malfunction
  • Malfunctioning catalytic converter
  • Ignition coil issues
  • Spark plug issues

Troubleshooting a Check Engine Light [ Problems & Solution]

Troubleshooting a Check Engine Light

Problem 1- Loose gas cap

The gas cap should be closed properly to avoid leakage of the gas. If the cap is loose, the system will detect it as a leak. So, you may experience a check engine light that is on, but your car is running fine.

The system in modern vehicles like Toyota, Honda, Chevy, and others is very sensitive to leak diagnosis. A loose gas cap may cause the leak, and the check engine light may start illuminating.


  • First, check for any cracks or signs of wear and ensure the gas cap is tightened securely.
  • Once done, reset the check engine light. You can do this by disconnecting the car battery or using an OBD-II scanner. If you use an OBD-II scanner, I recommend the FIXD Bluetooth OBD2 Scanner. It is easy to connect and use.
  • After resetting, drive your car for a few miles to see if the light comes back on.
  • If the light comes back on, your gas cap likely needs replacing. Purchase a new one from an auto parts store and install it.
  • Ensure the new gas cap is tightened securely, and try driving again.

Problem 2- Oxygen sensor malfunction

An oxygen sensor in modern cars measures the amount of oxygen in a car’s exhaust. It sends that information to the engine’s computer. If the sensor malfunctions, the computer may receive incorrect data and assume the engine is running poorly. So, it may trigger the check engine light even if the car appears to be running fine.


  • Start by scanning for error codes. Use an OBD-II scanner to check the error code.
  • Check the oxygen sensor to see if the error code is for the oxygen sensor. Look for any visible damage or contamination on the sensor. Replace it with a Denso 234-4622 Downstream Oxygen Sensor if necessary.
  • If the sensor appears dirty or contaminated, clean it using a sensor-safe cleaner.
  • Also, check the wiring connected to the sensor for any damage, corrosion, or loose connections. Replace or repair as necessary.
  • If the sensor is faulty or damaged beyond repair, replace it with a new one.
  • After making the necessary repairs, use the OBD-II scanner to clear the error code. Then take a test drive for a few kilometers to ensure the problem has been fixed.

Problem 3- Malfunctioning catalytic converter

If the catalytic converter is not working correctly, it can cause the emission levels to rise, triggering the check engine light. A malfunctioning catalytic converter can cause the check engine light to illuminate even when the car is running fine. You need to fix the catalytic converter issue to turn off the light.


  • Get a diagnostic scan of the vehicle to determine the specific code(s) triggering the check engine light.
  • If the code(s) indicates a catalytic converter problem, visually check the converter for signs of damage or blockage. If the converter looks damaged, it needs to be replaced.
  • But if there is no visible damage, use an infrared thermometer to check the converter’s temperature before and after the catalytic converter. The converter is not functioning correctly if the difference is less than 100 degrees Fahrenheit.
  • Replace the malfunctioning catalytic converter with a new one. Once done, clear the codes and reset the check engine light.
  • Finally, take your vehicle for a test drive to ensure that the problem has been resolved.

Problem 4- Ignition coil issues

The ignition coil ensures the spark that ignites the fuel in the engine. The coil malfunctions can cause misfires and affect engine performance, even if the car is still running. It can trigger the check engine light to alert the driver to a potential problem.


  • Run a scan for error codes using OBD 2 scanner.
  • If the scanner shows codes for the ignition coil, check the coil for any signs of damage or wear.
  • Also, check the connections and wiring between the ignition coil and the spark plugs to ensure they are secure and not damaged.
  • Then take a multimeter to test the resistance of the ignition coil. If the readings are outside the manufacturer’s recommended range, the coil may need to be replaced.
  • If the ignition coil is problematic, replace it with a new one.
  • Once replaced, clear any trouble codes using the OBD-II scanner.
  • Test drive the car to ensure the issue has been resolved.

Problem 5- Spark plug issues

A spark plug issue can cause incomplete combustion. It may lead to increased emissions and potentially damage the catalytic converter. The engine control module detects it and may illuminate the check engine light as a warning, even if the car is running fine.


  • Locate the spark plug; usually, spark plugs are in the cylinder head of the engine, threaded into the engine block.
  • Once located, check for wear and tear on your spark plugs.
  • If the spark plug is damaged, replace it with a new one.
  • Check the spark plug wires for any damage, and replace them if necessary.

How to reset check engine light?

If the problem has been fixed, resetting the check engine light is a simple task. To reset the check engine light, follow these steps.

  • Before resetting the light, you should diagnose and fix the underlying problem. Otherwise, the light will reappear.
  • Once the issue is resolved, disconnect the negative cable from the battery terminal. Leave the battery disconnected for 15 to 20 minutes. It will reset the car’s computer system and clear any error codes.
  • Finally, reconnect the battery’s negative terminal and start the engine. If the problem is fixed, the check engine light should be off.

Possible risks of driving with the check engine light on even if the car is running fine:

Driving a car with an illuminated check engine light can be a sign of several risks. It may cause engine stalls or damage, or decreased fuel efficiency. The light may be illuminating due to a minor or severe problem.

It may damage your emission system. Whatever the cause, it needs immediate attention to prevent costly repairs. So, driving with the check engine light on is not good.


Can I pass a smog test if the check engine light is on but the car is running fine?

No, a vehicle usually fails a smog test if the check engine light is on. It does not matter how well it seems to be running. The check engine light indicates an issue with the vehicle’s emissions system. So it is essential to have it fixed before passing the smog test.

How long can I drive my car with the check engine light on before it causes damage or affects performance?

It cannot be determined because the check engine light illuminates for many reasons. If the issue is minor, you may drive hundreds of kilometers without a problem. But if it is illuminating due to severe issues, you may not continue driving for long. So, diagnosing the cause and fixing the check engine light problem as soon as possible is essential.

Is it safe to use an OBD scanner?

Yes, it is generally safe to use an OBD scanner. However, it is essential to follow the manufacturer’s instructions carefully. You should avoid using it while driving and disconnect the scanner before repairing it. Also, be cautious when accessing sensitive information on your vehicle’s diagnostic system.

How much is to fix a check engine light issue? The cost to fix a check engine light issue can vary depending on the underlying problem. It can be due to a loose gas cap or as complex as a faulty sensor or catalytic converter. A diagnostic test can cost anywhere from $50 to $200, with repair costs ranging from $100 to $1,000 or more.

In the end, You can check the oil pressure, because low oil pressure may turn on the check engine light.