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Message: <blockquote data-quote="MyLittlePony" data-source="post: 1453677" data-attributes="member: 11359">I have a convertible. I rarely use the AC. A while back, I discovered that when I do use the AC, the engine overheats. No CEL though. This problem has since been corrected, and now blows ice cold with no overheating. HOWEVER, it now will randomly turn the CEL on, only to extinguish itself a few seconds later. Car behaves normally though.When I run the codes, AC off, I get:511 511 172 628 172 628AC on:511 539 511 539 1 172 628 172 628511 - EEC Processor Read Only Memory (ROM) test failureCar has a chip172 - o2 Sensor circuit indicates system lean (right side)Both O2 sensors have been replaced, and still has this code628 - Converter clutch Lock-Up errorCar has a shift kit on it.539 - System shows voltage at PCM pin 10  Is A/C on ?Yes.Basically all of these codes have existed within the system for years, and never once have thrown a CEL. One would think that whatever is now setting it off would be enough for the code to be stored somewhere, right? And it only turns on when the AC is running. A new compressor and drier were installed, but I can’t see how an AC system would throw a code, and if so, what?</blockquote>