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Old 11-19-2019, 10:38 AM   #3
JRTJH
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Join Date: Mar 2011
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A couple of things come to mind.

On some code readers, the codes are erased when the code reader is disconnected (or through a key stroke on the reader).. So, it's possible that you can read the codes, on disconnection, the codes are erased, you take the vehicle to the dealer and there are no codes in memory.

There are some "temporary codes" that illuminate the "check engine" light, but when the condition resolves, that code is corrected and the "check engine" light turns off. One such condition is the oil pressure. At startup, with low oil pressure, the "check engine" light might turn on, then, after the truck warms up, RPM increases and oil pressure also increases, the light may go out. This was a prevalent issue with older VW air cooled engines...

Anyway, without knowing more than a "very basic, the light comes on" and no description of what codes, what system, what circumstances, it's a "wild guess" to give any answer.

Much more information is needed to even understand what you're describing.
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