* Possible causes:
* MAF sensor is dirty or obstructed
* MAF sensor is damaged or faulty
* Wiring to the MAF sensor is damaged or loose
* ECM is faulty
P0171 - System too Lean (Bank 1)
* Possible causes:
* Vacuum leak
* Fuel injector is clogged or faulty
* Fuel pressure is low
* Oxygen sensor is faulty
* ECM is faulty
P0174 - System too Lean (Bank 2)
* Possible causes:
* Vacuum leak
* Fuel injector is clogged or faulty
* Fuel pressure is low
* Oxygen sensor is faulty
* ECM is faulty
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
* Possible causes:
* Catalytic converter is damaged or faulty
* Oxygen sensor is faulty
* ECM is faulty
P0422 - Main Catalyst Efficiency Below Threshold (Bank 1)
* Possible causes:
* Catalytic converter is damaged or faulty
* Oxygen sensor is faulty
* ECM is faulty
P0442 - Evaporative Emission Control System Leak Detected (Small Leak)
* Possible causes:
* Gas cap is loose or damaged
* EVAP canister is damaged or faulty
* EVAP purge valve is damaged or faulty
* Fuel tank is damaged or faulty
* ECM is faulty
These are just a few of the possible causes for the check engine light to come on in a 2004 GMC Envoy. If you are experiencing this issue, it is best to have your vehicle diagnosed by a qualified mechanic.
