I was working on my air conditioner a couple days ago. The next time I got into my truck to go somewhere the check engine light was came. As far as I can remember there was nothing I did that defiantly would have caused it to go on...
I put my OBDII tester on it and it is showing no DTC codes.. However, it is showing two I/M status faults: "Evap Sys Mon" and "Oxygen Sens Mon" both are showing as "incomplete"..


I think this could be because I did something that momentarily dropped the power to the OBDII system causing it to reset.... (I was in the fuse/relay box to bypass the compressor relay... I don't remember accidently shorting the wrong thing out but you never know.. If so the thing I need to do is just drive it until the light goes back out


Does this sound right? Is there something I should check?

 

I would try to reset it and see if they come back. Then work from there. It is very possible that you bumped some thing.

 

That's the problem... My ODBII scanner says there is no DTC code to reset.. It shows no DTC codes found. If I understand the instuctions that come with my ODBII scanner correctly, It shows that the only problem is that there are 2 I/M conditions (not codes) causing the check engine light to be on. And, if I understand correctly, the vehicle has to go through a "cycle" of driving conditions before these items are considered properly tested... Until the vehicle has completed this "cycle" of driving, the tests are considered "incomplete". Once the "cycle" of driving has been completed, If all the parameters are correct, the computer will cause the check engine light to go out. If once the "cycle" of driving has been completed, some parameter is found to be out of spec, The check engine light will remain on and It will then throw up a DTC code. As I understand the way it works, it is done this way so people who live in a state that requires emission testing cannot cheat the testing by reseting the OBDC right before going into test facility.

 

That is accurate.
why it would all of the sudden change with out a code is weird.

 

Yes it is weird... but I must have done something to reset the monitors.. Today I went out. The check engine light was on.. when I got in to go home... the light was out. The monitors must have completed the cycle.

 

what if the light doesnt reset on its on. is there a way to do it manually?

 

Go to auto parts store and borrow code scanner. Or disconnect the battery for about 10min or so. If it comes back on the only way to turn it off is to fix the problem.

 

"Soft" reset
- Most code scanners will allow you to "clear codes" easily. The history usually remains, but the code clears unless/until it is reset.

"Hard" reset
- Disconnect BOTH leads from battery.
- Touch them together for 30 seconds (this drains the KAM caps)
- Reconnect the battery.

N.B.: The latter method should flush everything - including "adaptive learning. This means that there may be a brief "relearn" period for a few days, since you reset the ECU back to factory clean...