My 2001 H-6 headlights flicker and today the check engine light came on. O'Reillys says battery and alternator tested O.K. All ground wires are free of corrosion far as I can tell. Any suggestions?

Welcome to the USMB.

 

Did O'Reilly's pull the codes from the ECU for you?  I would suggest having them do that and report back on what codes (if any) are stored in the ECU.

I will see about the codes first thing in the morning.

Her is the deal. The alternator was bad in that it was over charging the battery and was being kicked off on. The  alternator primary ground wire had been burnt through and was no longer usable. Tech code showed an unrelated fuel line problem that was corrected by the gas cap being tightened. That accounted for the warning, check engine light.

 

So basically the alternator and some wired had to be replaced. There was plenty of warning time had I of known the cause. Unfortunately the repairs were pretty  steep at around 6 bills. Glad to have the car fixed how ever as I never would of seen the burnt wires under the shade tree.

Glad to hear you got the car back and it's working, even if it cost you a bit.