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Could be any number of reasons. During a driving cycle(s) the computer checks its sensors if operating within spec, if not, the CEL comes up. If during x number of driving cycle(s) the computer determines the sensor(s) operating within spec, it will clear out that code.If a sensor isn't operating properly within spec but still between driving cycle(s), it will log a pending code and if it continues it will go to a full fledged CEL.
The reason for intermittent CEL can be many:heat, weather, humidity, wiring/connectors, degrading/faulty electronics (ecu, coils, resisters, boards, etc.), fuel related*, elevation, vibrations, goblins, possessed spirits, UFOs, mischievous people.
*I have an intermittent CEL random misfire code (p0300?) that occurs when my gas is 1/2 or less full. Car will start up the 2nd time, everytime, but will stumble and run rough..then the CEL will come on about 15 seconds later. If start car again within few hours with 1/2 or less of gas, car starts right up. If I keep the gas more than 1/2 full, it will go away after about a week of driving and the car starts and fires up after the 1x attempt within couple cranks. I've pretty much narrowed it down to fuel delivery (vs ignition), and one of these years will replace the fuel pump, and its pressure regulator (and filter). This is just an example of an intermittent CEL where in my case, if there is more gas (1/2+) in the tank--somehow-someway (gravity/pressure...The Force--I dunno..lol) I don't have this issue.
Last edited by l33ch; 01-15-2018 at 11:29 PM.