CEL Turned Itself Off Without Reset?

[numberforty1]

Here's something we all wish would happen, but I didn't know it was possible.

Had 2 codes - PO135 for O2 sensor and PO446 for EVAP.

Replaced O2 sensor and reset ECU. After about 30-40 miles the CEL came back on, as expected, since I had not addressed the EVAP issue yet.

After driving another 30-40 miles, the CEL suddenly went off, by itself, right while I'm driving down the highway.

Its been another 200 miles of highway and city driving, still no CEL.

Is the CEL ever supposed to just "go away" like that? I thought the CEL was supposed to stay lit until the ECU gets reset.

Any thoughts on how replacing the O2 sensor might have an impact on the PO446 code as well? I know, wishful thinking...

[thaharlequin]

I can't speak to OBDII, although I don't know why it would be different than OBDI, but yeah, if the problem isn't detected within a certain interval due to normal operation of whatever it is the ECU is monitoring it'll drop the CEL. I had a heated O2 sensor I had wired up incorrectly and if the load on the engine was high enough to make enough heat for the sensor to start working the ECU would drop the CEL like clockwork.

[numberforty1]

Thanks, I didn't know the CEL was continually monitored by the ECU like that.

This is really getting into the details, but, anyone know of a database containing information related to the timing/distance requirements that set off codes in the ECU from various sensors?

[thaharlequin]

The closest thing would be the FSM. IME it's not distance so much as satisfying certain conditions. In my car for an O2 sensor CEl to be thrown I would need to let the car warm up, then accelerate at near WOT to a certain speed and keep it there, so if I managed to not reach those conditions by modifying my driving style then I could drive around w/o the CEL coming on for my worn out O2 sensor for well over 100 miles. That said, OBDII is a bit more finicky, but if you had the FSM I imagine you might be able to figure out what conditions tripped a CEL and how you could avoid those.