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What could trigger my car's 'check engine' light?

Front half of a black Lexus car.

(Photo: Flickr/M 93)

What could trigger my car's 'check engine' light?

July 29, 2013

Q: I have a Lexus GS300, and the indicator light in the shape of an engine just came on. What does it mean?

A: The light you are referring to is commonly known as the "Check Engine Light." When the light comes on, this is the on-board computer's way of telling you it sees a problem in one of the monitored systems. Unfortunately, there are literally 100's of parts and sensors monitored in various systems, and to take a "guess" as to which part/sensor in which system is faulty is virtually impossible.

However, if you are not experiencing any drivability issues such as skipping, stalling, shifting problems, etc., then more than likely the problem is going to lie in the EVAP (emissions control/recovery) system. Very commonly, this could be a faulty, loose or missing fuel cap -- or even putting fuel in while the vehicle is running can set the light.

Check the underside of the fuel cap for any cracks or signs of defects that may keep the cap from sealing correctly. If there are any doubts about the inspection of the cap, replace it. Keep in mind you will need a cap that meets OE specs. Those "universal" or "locking" fuel caps sold at after-market parts stores do not meet OE specs and will not seal the system correctly.

Check the top of the fuel filler neck for any signs of damage or debris. Check under the vehicle, around the rear areas of the vehicle, looking for any vacuum lines that have dry-rot cracks, holes or loose/missing connections.

If these all check out, then the best thing to do at this point is to have the Diagnostic Trouble Codes (aka "p-codes") read from the on-board computer. These p-codes are what is used as a "starting point" for the diagnosis and will tell which part/sensor, in which system, has the fault.

There isn't a mechanic on the planet that can tell you what is wrong with the vehicle just by "looking" at the Engine Light. Every single mechanic's first course of action WILL be to obtain the p-codes. It's standard diagnostic procedure. Once you get these p-codes you can more accurately and efficiently diagnose the problem and then make the correct repair. You can have these p-codes read FREE (except in California) at any local "big chain" part store (ie., AutoZone, PepBoys, Advanced, etc). If you still need help after retrieving the codes, you can bring them here and any of our techs can walk you through the diagnostic procedure.

As long as there are no drivability issues or shifting issues, then it should be perfectly safe to drive the vehicle. You should, however, have the codes retrieved at least to verify. If the problem does lie in the EVAP system, it is strictly for emissions control/recovery and will in no way effect engine performance in any way. If the problem lies outside the EVAP system, it may be a good idea to have it serviced as soon as possible, as prolonged driving with a problem in with the engine or transmission can make matters worse.

-- Answer from Ron Z., a Lexus technician on JustAnswer.

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