Check Engine Light AGAIN

[David C Klasse]

To those of you that have been following my posts, and others: Damn Check Engine light came on again yesterday (maybe it'll burn out soon and I wo'nt have to worry about it!). I am assuming that it's the 02 sensor again. The code was cleared the day before, and went away minutes later, but is back on. COuld the O2 sensor be working intermittently? Or was it because the error was cleared, then registered again in computer? ...which seems like most plausible explanation.
Another twist is that the check engine light came on last night when the ambient temp was about 45 degree F. I had heat on full and was driving about 50 mph. Well the Check Engine Light came on at this time, and the engine temp was well below the 80C, but had already been warmed up to operating temperature, and when I had the heat on and driving and cool air must have cooled engine down. 1. Is engine temp supposed to go that low, even with heat and cool air around? and 2. The check engine light went on when engine was below 80C, could this be a correlation to the 02 sensor registering as bad in comp?? Hope I didn't lose anyone on this! Thanks again to you all!

[Rocket]

OBD II engine management waits until it sees a fault still exist after two driving cycles before it is considered it a matured fault and turns on the check engine light. A driving cycle varies a bit from manufacture to manufacture, but basically it looks at warm up, cat. function, air injection, EGR ,various engine loads and oxy. sensor data. The full data picture takes about 20 minutes to complete the computer's view. If someone did short back and forth to work cycles they could go for weeks with out setting a check engine light. The only conditions that sets an immediate light are those that the computer sees as ,catalyst damage is emminent. If your misfire has gone away most likely you have an aging oxy. sensor code. The only fix is a new sensor. Good Luck!

[David C Klasse]

Thanks ROcket. I'll have them read the code again on Monday and if it's O2 sensor, which I'm sure it is, I'll have them go ahead and put the new one. Thanks for the help.
BTW, about 10 minutes ago, I unplugged battery to see if that would clear fault (not sure, but got mixed info on this in the search)... I'll let you know.
...Hooked battery back up and started car. CE light is gone! Not that it matters really, I'm sure it'll come on next time I drive the car and I'll take it in. Thanks for all the valuable info Rocket.

[David C Klasse]

OK, Since I reset the check engine light yesterday (unplugged battery), light has still not come on, it's been 3 driving cycles. ANyway, I want to know if the 02 sensor is indeed good, but something else is triggering the 02 sensor or making a fult in the computer with o2 sensor. Could something be causeing the 02 sensor to register as bad? Or when the computer says 02 sensor is bad, it's bad, no questions... ?

[stevebfl]

It's my experience that very few oxygen sensor problems are the sensors them selves, especially if they don't have 120k on them. There are numerous fault codes for O2 sensors. Proper diagnosis requires testing based upon what protocol is failed. All the parameters that are monitored by the control unit can be monitored by testing. Min and max voltages, rich lean switching time, lean rich switching time, cycles per second. Sometimes the time to the first switch is monitored and fails due to the engine not being heated properly.

Check your code and follow the diagnostics based upon the number.