There's a Technical Service Bulletin written for these vehicles, the wires in the fan connector go bad. They make a replacement connector. I hope this helps. Take care?
SOURCE: 2003 envoy sounds like a jet i just replaced fan
IT MAY BE THAT THE CLUTCH IS LOCKED UP BY DEFAULT, SO THE ELECTRONICS TO THE FAN CLUTCH ARE MOST LIKELY THE PROBLEM.
SOURCE: 3 codes present on my 2002 Envoy. p0172, p0420, and p0340,
Track the p0340 first camshaft timing is very important and could possibly cayse your other codes,Call your local GMC dealer I think I remember a bulletin about this code.
SOURCE: 2002 GMC envoy slt 4.2
Sounds like you may have disconnected the battery. Does the rough idle only happen when the defroster or A/C is on? If so clean your throttle body with throttle body cleaner with the battery disconnected. When finished, drive it for 5 or so miles with the defroster / AC off to let the computer relearn the settings. This will solve your problem.
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