I FINALLY got my CEL cleared today. I took it to a Chevy dealer last week, but after wasting my day waiting, they decided they didn't have the software to troubleshoot my problem, so they referred me to another place town, Warrenton Foreign Car.

They determined that both my OEM and new NAPA EGR valves suffered the same failure! Something about a high resistance across one of the circuits (in the M-Ohms versus K-Ohms.) My NAPA EGR was well within the three year warranty, and knowing Warrenton Foreign Car very well (it's right across the street), they exchanged my EGR valve in just a few minutes. Warrenton Foreign car then verified that the resistance of the new EGR matched their shelf unit, installed it, ran a few drive cycles and cleared the code.

All of my A-B comparisons were based on the assumption that the NAPA EGR valve was good, or at least if it wasn't, it would return a different set of symptoms. Because it failed in the same manner as the OEM unit, swapping them out proved nothing. D'OH!!!

I suspect, but cannot confirm, that both EGR valves were somehow damaged by the bad battery and corroded battery cable that I replaced after the NAPA EGR threw a set of codes. I had strange transmission vibrations, my GPS refused to power up, and I had multiple codes when I was returning from my vacation in Georgia last winter. Other than this CEL I haven't had any electrical problems since I had the charging system repaired.

Keep your battery and cables in good condition, or else!