Hardedge

I had a stored code on my stock EM1's ECU for the primary O2 sensor, but while working on the car the battery was disconnected and therefore the ECU reset. The problem is that I'm insurance shopping right now, and so I don't have liability an in turn cannot drive the car out on the road in an attempt to bring back the CEL. However, I know the problem is still there because the engine is sucking up fuel like there's no tomorrow. My question is, will the MIL indicate pending codes when running a DIY test, or just stored codes? Also, before you say "if you knew it was the O2 sensor then just replace it", I still want to double check the ECU and also make sure that it's still the only code.

Luserkid

Sometimes the ecu can catch codes that are barely showing. But other times it wont, it really depends on the code. If the ECU does not see it as a big problem (like o2 sensor), it wont throw a code.

slowcivic2k

Yes, any Honda OBD1 and all OBD2 will not illuminate a pending code, but it will show up if you put it into diagnostic (paperclip) mode. These codes will stay there for 40 full operating cycles, or until the battery or ECU is disconnected from power, then of course the codes will clear.

The ECU has a suspend function that will retest a particular system. (say the O2 system) If it fails the first time, it will throw a pending code that you will not see, the next time it fails, it becomes a hard fault, and the CEL will come on. That procedure is used because the abnormal conditions that cause it to happen may have been spontaneous, so instead of condemning the sensor/system, it retests to ensure its an actual failure.

There is one exception to the rule. If the failure causes emissions to increase 150% over the federal limit, the CEL will come on for the duration of the problem, go out, and then come on solid with the problem system.