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I was discussing the behavior a car's check-engine light. When a check-engine light comes on, can the check-engine light turn off on it's own or does the check-engine light always require manually clearing to turn off?

Can you provide a canonical example(s) of when the check-engine turns off on it's own and another example where the light stays on until manually clearning?

  • I was reading a P0420 Catylist not running at proper level and I was told to check the O2 system I did and the small hose that runs to the bottom of the engine that the air from the engine leaves through I believe and it was not attached so my exaust was running out infront of my car I replaced the hose and am hoping that fixes the issue but do I get it cleared or is there a certain timeframe I have to run my car for before it shuts off – Dylan Lintz Jul 20 '18 at 18:50
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The light can clear itself. For instance if you leave the gas cap off when refueling, the light will come on and a code will be entered into the computer memory. Once the cap is put back on and a drive cycle occurs where the computer can detect that everything is good, the light will go off. The light will go off, but the code will remain in the computer until cleared (for whatever reason, such as lack of power or being manually cleared by a handheld device).

Obviously, if the issue is something which needs a mechanic's touch, the light will not go off. Mechanical beasts don't cure themselves. Anytime something gets beyond the thresholds set inside the computer, the light goes on. This can be anything from a bad heating element of an O2 sensor to a bad circuit or lack of continuity in a MAP sensor. Each throws a very distinct code which will not shut itself off until the issue is fixed.

  • I was wondering about if it was possible for the CEL to turn off but the code to remain. I forgot to add that into my question but your answer addresses that. – syn1kk Oct 27 '15 at 22:50
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If the condition that caused it to come on is a minor fault, and stops occurring, then yes, it will clear itself. If the condition indicates a larger problem, then it will stay on until cleared manually.

A good example of a condition that will clear itself, is low brake fluid. I experienced this myself when I would only have the light come on when stopping hard over time, which triggered the sensor. After the force of stopping subsided, the sensor state returned to normal, and the code / light was cleared. Also, timing issues will usually clear themselves if they do not persist.

  • You'll notice, though, that it's your brake light which will come on, not your CEL when the brake fluid is low. – Pᴀᴜʟsᴛᴇʀ2 Oct 27 '15 at 22:23
  • Not (always) true. In my vehicle the CEL came on and when I checked it, it gave a code for the brake fluid. – Ian Wizard Oct 27 '15 at 22:30

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