My 2002 envoy started running rough with a p302 code. I replaced the ignition coil but it still running rough and using a lot of fuel. Check engine light is on.
Check to make sure the coil has the proper voltage and is receiving the firing signal..
Had broken wires in the engine harness that was causing the same problem..Dealer said it was a known problem on older models.. the wire is undersized and the flexing from engine vibration causes the problem.
GM refuses to acknowledge the problem.... Said "Out of Warrentty"
