irregular idle..zzp downpipe..help??

Car in question is '00 GT 9x,xxx miles. Had been running fine until the other day. Now I'm experiencing a strange idling issue. First off, about two weeks ago or so I had the ever popular 2.5 inch high flow downpipe put on from zzp(not impressed). They couldnt get the old o2 sensor off the old cat he claimed so they plugged the hole with an old one and cut the wire, so obviously it set the SES light. Anyway, fast forward to now and im having an idling problem when the engine is hot. It seems to run fine and doesnt stall it just does this idle drop and revs a bit dont know how else to explain it. Could it be a backpressure issue or does something else need to be checked?? Ive run fuel injector/system cleaner several times before this even started. Thanks in advance.

ps no mods other then the downpipe

Not 100% sure if this could attribute to it, but through some STRANGE "coincidence" my car started to throw code P1133 almost immediately after I installed my downpipe. I think either the downpipe is messing with flow (not likely) or the guys that installed it screwed the o2 sensor during installation (I have NO idea how...too much RTV at the exhaust flange?) Anyway code P1133 is the front o2 not switching fast enough...maybe they screwed your front o2 sensor and it's not fueling the engine properly (the HO2S front o2 sensor is actually important, it measures o2 levels and changes your lean/rich fuel mixture accordingly). Find a code scanner and see what pops up is my suggestion.

Yes, I agree, if the SES light is on, best to get it scanned before we go any further. Autozone or Advance does it for free. Don't trust their on the spot diagnosis. They are usually trying to sell parts, and none of them really know anything about cars.

The SES light is because there is no rear o2 sensor plugged in, just an old one to plug the hole (had the wires cut) I'm getting a new one soon. From what I've read it seems my issue could be due to a dirty IAC. What do you guys think?? Whats a fair price to pay to have it cleaned out??

I couldn't tell you about the IAC, but just because your car is throwing code 0420 due to the rear o2 doesn't mean it's not throwing other codes too...can't rule something else out until you scan...

Possible be a bad IAC. Couldnt hurt to take it out and clean it up.