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CHECK ENGINE LIGHT – WHAT DOES IT MEAN?
by Lance Wright

BACKGROUND INFO
All cars and trucks come with indicator & warning lights on the dash panel. Some simply provide information and/or indicate normal operation such as the green arrows that blink when your turn signals are on or the “brights” ie. high beam indicator, to let you know your headlights are on high beam.

Federal regulations required all vehicle manufacturers to have certain core trouble codes in common with their computer (EEC) controlled vehicles. These standards include universal data interfaces, trouble codes, as well as standard component terminology. These standards are known as OBD-I and OBD-II (OBD stands for On-Board Diganostics). Due to this standard, many auto parts stores read out the OBD trouble codes free of charge (in hopes they can sell you the necessary parts for your auto repair job).


THE CHECK ENGINE LIGHT

Located on the instrument panel of all modern cars, the “Check Engine Light”, also sometimes called the, “Service Engine Soon” light or more technically known as the Malfunction Indicator Light or lamp (MIL) is used to inform the driver of a failure in the engine or transmission control system that could affect vehicle emissions. When this light comes on it does not automatically mean that you are facing a huge auto repair bill. It could simply indicate a glitch in one of the sensors that feeds the electronic engine control unit (EEC). This light is an output of the EEC control module and, prior to 1996, is used to display diagnostic trouble codes which aid a mechanic in carrying out an automotive diagnostic and repair procedure. From 1996 and on, the trouble codes (called OBD-II or just OBD2) were standardized on all vehicles sold in the United States. There are about 500 generic OBD-II codes as well as dozens of manufacturer specific trouble codes (Ford, GM, Chyrsler, etc).

The MIL is located in or near the instrument cluster. It will illuminate, as a bulb check, when the key is turned to the run position with the engine off. After the engine is started, the lamp will remain on for a few seconds then go out. If the engine control module detects a failure in the emission system, the lamp will be illuminated to inform the driver of the need for service in which case you should visit your nearest auto repair shop for analysis.

Government regulations have affected the operation of the check engine light as of 1996. OBD-II regulations require that the lamp illuminate when a failure in the vehicle emissions system causes the vehicle to exceed one and a half times the federal emission requirements. In addition, the vehicle MIL must flash to warn the driver of a catalyst damaging misfire.


INTERMITTENT CHECK ENGINE LIGHT

An intermittent check engine light situation is often very difficult to diagnose since the light will go on and off thus making it hard or impossible to capture the underlying trouble code. On some cars and trucks the check engine light will go off when the problem self-corrects or goes away (eg. one of the sensors now measures a normal reading). On others you’ll need to use a scan tool to turn the light off manually but, of course, only after the necessary auto repairs have been performed. Still on other cars, the underlying problem (if there even was one) may self-correct and yet the check engine light will still stay on; this requires it to be manually cleared. Due to strict California emission standards, some vehicles may set a evaporative emission OBD-II trouble code even if you leave the gas cap loose. If the check engine light light goes out, the OBD-II code usually will still be stored in the EEC computer as long as the battery in the car stays connected. But on some of the newer vehicles, the OBD-II codes stay in memory even if you disconnect the car battery.

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7 Comments on "Check Engine Light"

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Ha huynh
2 years 8 months ago

Please,help me code p0402 exhaust gas recirculating flow excessive detected what part I need replace! Thank you!

don pendergrass
2 years 6 months ago

how do you reset the check engine light on a

1998 Cheep Cherekee

david
1 year 4 months ago

my check engine light does not come on at all no time i checked all fuses replaced instrument cluster same

obeto
1 year 3 months ago

2005 PT CRUISER trouble code p0073

Kenny
3 months 11 days ago

I have a 1994 Chevy cavalier station wagon with a 3.1v6 front wheel drive car I replaced the crankshaft position sensor and Map sensor battery and the car started right up had no problems with it it ran like a champ after that I put a CD player in it and it ran good for a couple weeks and then all of a sudden it started acting up and the lights started going deem when I hit my breaks and when I excelerate the lights get bright and then on top of that the car started dieing often but with a Jump it would attend run for awhile and then it would die and not start with a Jump.then replaced map sensor and it ran awesome drove it.bring back home parked it shut it off waited for a min and try to start it it fired right up a couple times no problem.went in side for a while then me my wife get ready to go to the store the car starts but dies and won’t start back or have a charge at all

Kenny
3 months 11 days ago

Any idles to what might be wrong with it

John Lavallee
26 days 7 hours ago

Hi. I have a 2003 Nissan Altima with the Service Engine Soon light on. I had the valve cover replaced because it was leaking oil. I am toping up the oil quite often, such as weekly. I noticed a smell like burnt rubber coming through the vent, and not sure what is the cause?

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