Why would a check engine light be on?

Answer 1

The check engine light comes on and stays on when a problem is detected by the self diagnosis system of your vehicle. A flashing check engine light is an indication of a severe misfire or problem that is likely to harm your engine. Immediate diagnosis and service is needed. Generally, the problem is in the emissions or something that is affecting the effectiveness of the emissions, but other causes can trigger the light to come on, too.

To determine the cause, the vehicle will need to be taken to AutoZone or a shop that has an OBD2 scanner, or for 1995 and earlier models, an OBD1 scanner, While a scanner will not give you the magic answer, it will give you a code that narrows down the problem area. Or you can buy a handheld computer called a Diagnostic Code Reader that will do the same thing and may be cheaper. Plug the tester into the OBDII port inside the car. You'll find the port under the dash to the right of the steering wheel column. Many auto supply stores will loan you the test unit at no cost or will run the test for you.

Some specific causes for the check engine light to be on are:

• Gas cap left off, not tightened, not installed correctly, or needs to be replaced
• 02 sensor
• Engine misfire - error might clear itself after 3 complete warm-up cycles; not restarts, but warm-ups
• Problem in the emission system, including clogged exhaust or the catalytic converter not functioning properly
• Dirty/clogged air filter
• Dirty MAF sensor
• faulty sensors in exhaust system

There are pages and pages of items that can cause a check engine light to come on. Having it checked with a scan tool will narrow down the problem area.

More comments from Answers.com contributors:

• Not all systems are able to be read by your local auto parts store. For '95 and earlier cars, OBD I is used and can NOT be read by AutoZone or Advanced Auto. A simple paper clip can be used to jump 2 ports on the diagnostic port and have the Engine light flash a certain sequence. This will then tell you the code to refer to for troubleshooting.
• First rule of thumb is to check your gas cap; if it has been screwed on wrong, that will cause your light to turn on. Second, check your engine oil, you might be running low. Next, take it to a mechanic because it could be something worse. It could be anything; broken sensor, engine problems...take it to a mechanic you trust. Or if you still have warranty, take it to the dealer.
• The "check engine light" is by far one of the most misunderstood technological advances by the public. I am sure I will revise this as time goes on as it is an in-depth understanding for the public. It is a warning light that is illuminated when there is a problem with the EMISSION SYSTEM only. Emission system being the pollution control system. Don't get a hard on against it as it is a good thing once you understand it. One point that was brought up a a recent meeting of technicians was that the amount of hydrocarbons is greater when the gas cap is left off than when the engine is running. Hydrocarbons are part of pollution emitted as gasoline evaporates. Going a step farther, one facet of the emission system is the "Evaporative" portion. This is when the fumes from the gasoline are leaking from the system into the outside air. This is one part of the emission system that can trigger a check engine light. I would say that about 7% of the vehicles that have a check engine light are the result of a loose or inadequate gas cap. But understand that many scenarios are possible with the "check engine light" The vehicle's powertrain computer (note that some vehicles have 17 different computers) will run a series of self-tests. They will only run under certain criteria. And they can be vastly different from manufacturer to manufacturer. Some self tests are not run until preceding ones have run successfully. So if there is a problem in one particular area that is preventing another self test from running, you can have a situation where one problem is fixed, but another still exists. If you fix a problem and drive the car through a drive cycle that sets the monitor (or self test) the light will go off as it passes that criteria that triggered it in the first place.

  After 1996, the auto industry went to a idea called OBD II (on board diagnostics). This was to get all the manufacturers onto a similar plane for troubleshooting and powertrain control. While they still differ vastly, many corrections and adaptations were made for technicians to better fix the check engine light problems. Prior to this there were so many different and poor troubleshooting data from a check engine light problem that resolving the problem was much more difficult. Many early warning light of this nature were set to illuminate based on mileage. An Oxygen sensor was one of the things that were meant to be replaced when that mileage was hit. This is much like many current "Change oil lights", that are set based on a preset mileage.

• Check engine lights involve the engine's electronics and/or electrical system, since they are interrelated. My '97 Sable's recent check engine light signified a problem with the camshaft sensor, which had to be replaced. Important, since it works with the ignition system, and while AAA is good to have, I hate being towed. It's a tricky repair, and like most manufacturers, Ford has the system designed to mandate a particular repair tool that's expensive to buy. The job's a pain anyway, and can be tricky, so it's best to have a certified mechanic you trust do it (I do not, repeat, do not, like dealerships for a variety of reasons). Never ignore a check engine light; generally easy to computer diagnose (but prepare to pay for an hour of diagnostics). All dash lights are potentially bogus -- sometimes these things take on a life of their own, signifying nothing, but it's foolhardy to assume that. Hope this helps.