My check engine light came on a couple weeks ago and I brought it to the shop. I pulled a code on it first; P0121. Shop said the throttle body and valves where really dirty and they did a fuel service on it. Light is now off, obviously they cleared the codes, but now idles really rough. Sometimes it will remain idling at 2100 in park. I took it back a couple of times and they owner said I need to take it to GM to have the ECM re-flashed/re-programmed. Has anyone had this issue with their Yukon/Tahoe? I'm really frustrated because I know the dealer is going to charge me $100 for a diagnostic fee even though I think I already know what it needs.
Any feed back would be cool.