Random CEL on VWs/Audis

I think I've figured out my own Check Engine Light coming on and off seemingly at random. I did have it check and my indy Audi mech, and he said there was nothing wrong with my O2 sensor or the cats. The CEL stayed off for a while and I realized I was only going to BP gas stations. I was also only getting the highest octane available and it didnt have ethanol in it. I am not sure if its BP gas or if its the lack of ethanol or the combination but since I've been staying away from MotoMart gas stations and sticking to BP and non-ethanol high octane gasoline the CEL hasn't come back yet. Anyone else notice this at all? Any other crappy gas stations or any other good gas stations? (This is for my '00 Audi A6 2.7T Quattro.)

what codes is it thowing on?

2.7t's throw cels for a small cross breeze. Scan it, but don't be surprised when it comes back with all sorts of random stuff that doesn't make sense. I had a similar situation with by B5 S4 which started like this (random cel for misfires) and it eventually turned into a larger problem (massive misfires) that was one of the ignition modules going bad.

I believe it was the O2 sensor(s), but my mech said that the O2 and cats were fine when he tested it and just cleared it for me. Then it came and went seemingly at random. It was off for weeks until I went to MotoMart for some gas and it came back on. I noticed that I was only going to BP gas stations so I've stopped going anywhere else and it hasnt been back since (another couple of weeks). Maybe the ethanol or crappy gas mixed with good gas would throw a code at the O2 sensors and would trip and clear when the crap would be under a certain mixture with the good gas. Just seeing if anyone else noticed this happening too?

You'll get O2 bank efficiency codes until the misfires are bad enough to cause a cel. Never had it happen with different kinds of gas, but entirely possible. You can check it with a vag-com watching the blocks for misfires and see if its all banks or just one.

Thats some good to know info, thanks Paul.

i have a stupid "Cat Effiency" code that keeps being thrown on, its the stupid Hi-Flo cat the last owner put on. I clear it out, hoping to drive atleast 1 drive cycle without the CEL popping back on, so i can get it smogged. No such luck. I even put an anti-fouling cap on it, like the caps for old V8 70's cars, trying to trick the o2 sensor... no such luck again...

Rabbit: Try running BP high octane w/out ethanol. That cat effiency code sounds fimilar. I cant remember for sure what the code was... Maybe it'll work...

I thought a CEL was SOP for a VW?

ya im thinking the cat is problem and i will need to get a stock one welded back on, unless my company could write some code to be flashed on my car to turn it off...

you could also get a VAG COM code reader for a laptop - like 5 bucks on ebay...

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=140335611254

Make sure you're not logging little misfires causing that code. It will throw that code first when the ignition amps are bad with the 3pin coils and when the coils are going with the 4pin ones.