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Check Engine Light



Be sure that the ignition is in the RUN position, engine off, when reading or clearing trouble codes

While UltraGauge can most certainly turn the Check Engine Light (CEL) off, your engine computer might in an instant turn the CEL back on. With most makes and models of vehicles, the CEL turns off and stays off. However some vehicles, like early GM models, turn the CEL back on quickly.

The OBDII standard defines the behavior of the check engine light and clearing codes and the CEL.  However, the standard only specifically describes "emission related diagnostic information"   Any trouble code which is not emission related, is technically not governed by the OBDII standard and the posting and clearing of such codes is at the manufacturers discretion.

Manufacturers create classifications of "trouble", like a hard and soft failure. A soft failure would be a sensor that intermittently goes full scale. A hard failure would be a sensor that stays full scale. Some manufacturers continually read such sensors and if a hard failure is found, the trouble code is logged and the CEL is turned on. This all happens about as fast as you can blink, so the check engine light may appear to remain on.

Also, some manufacturer's algorithms are such that they want to see the absence of the fault through several driving cycles before they consider the code clear.  Only then will the CEL be switched off and the trouble code removed. To potentially simulate this, the ignition switch can be repeatedly cycled from OFF to RUN to OFF. The number of times necessary is manufacturer specific. Its typically best to leave it in the RUN position for at least 5 seconds each cycle.  Other manufacturers may require more extension driving cycles.


Please note that some vehicles will ignore the command to turn off the check engine light when the engine is running.   However, all manufactures are required to respond to the command when the ignition is on, but the engine is off.

Note:  If it is your intention to switch off the check engine light and clear codes to then attempt to pass smog, you should know that many vehicles have additional parameters such as "Miles since CEL cleared",  "Miles traveled with the CEL on", "Number of warm-ups since trouble codes clear". "Distance since CEL cleared", "Engine Run time since CEL cleared", "Engine Run time since trouble codes clear", etc.   Smog centers which check these parameters, if your vehicle supports them,  will not be fooled by clearing the CEL and trouble codes.

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Article ID: 4
Category: Knowledgebase

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