Shared Knowledge

"Check Engine Light (CEL)"

23 March 2014



Summary: the CEL is emissions related. If vehicle is running ok and gas mileage has not changed significantly, can ignore light for a while. Could be something simple or require hooking vehicle to "scanner".



All modern vehicles have an Environmental Protection Agency (EPA) mandated emission control system.

Emissions control is done by using a small computer to monitor various sensors and adjusting the amount and timing of fuel to the engine.

If the computer detects something wrong with the emissions system, it turns on the CEL.


What to check, do:

Not all causes of a CEL are serious.

Sometimes the CEL will come on for a few days and then go out on its own.

At a minimum:

- Check that the gasoline cap is on tight. Unscrew it and then attach again.

- If low on fuel, fill tank.

- Try changing gasoline stations that you use.

- If you are used to squeezing just a little bit more into the gasoline tank at a "top off", back off.

- If you have been using a lower grade fuel than recommended by the vehicle manufacturer,  move up a grade.

- If the air filter has not been changed in a while, change it.

- Try adding some gasoline "dryer" to the tank. All gasoline has ethanol in it and ethanol absorbed many more times water than actual gasoline. Since ethanol is heavier that gasoline it eventually sinks to the bottom of the gasoline tank and can be the cause of a CEL for a vehicle that has been sitting for a long period of time.

- If after driving for 40, 50 or more miles with multiple engine starts from dead cold, the CEL is still "on", take the vehicle to any place that will hook to a scanner (free at many places). Only by hooking to a scanner can the cause of the CEL be determined.

- You can borrow a scanner from some auto parts dealers or even buy your own but takes a while to get up to speed using these devices.

- In many states, a vehicle will not pass a safety or emissions inspection if the CEL is lit.


CEL reset:

Once your vehicle is hooked to a "scanner", the CEL can be reset.

- Note: if the CEL is reset on your vehicle, the vehicle's computer will not report that it is ready for emissions testing until after you have driven 40 - 70 miles. This means, you can not have the CEL reset and then hope to get the vehicle to pass an emissions inspection.

- Also note that every time, the battery is disconnected from the vehicle, the emissions computer is reset and thus will report "not read" if you try to have an emissions inspection done before you have driven 40 - 60 miles.


Scan Results:

Scanners connected to your vehicle should specifically highlight the problem with your emissions system.

The most likely cause of a persistent CEL is a defective oxygen sensor, of which there are a minimum of 2.

There is a small electric heater in each oxygen sensor that eventually burns out.

Oxygen sensors are expensive, $200 - $300 (question if repair shop wants more for sensor) but easily installed so one hour usually is enough labor costs.

If you are told you need a new catalytic converter, take your vehicle to another shop and have it scanned again. Catalytic converters are expensive to buy and install and designed to last the life of a vehicle or at least 500,000 miles.

If you are told you need a mass air flow meter, take vehicle to another shop and have it scanned again. Mass air flow meter or sensor is very expensive and seldom fails.

A fuel injection cleaning will not solve your CEL.



Ron - Shared Knowledge Home