Possible causes

– Intake air leaks
– Faulty front heated oxygen sensor
– Ignition misfiring
– Faulty fuel injectors
– Exhaust gas leaks
– Incorrect fuel pressure
– Lack of fuel
– Faulty Mass Air Flow (MAF) sensor
– Incorrect Positive Crankcase Ventilation (PCV) hose connection

Tech notes

If the P0171 is combined with the P0174 code, it’s very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replaced the air filter and clean the air flow meter. If the problem persist the front oxygen (O2) sensor may need to be replaced.
The P0171 code could also be caused may be caused by one or more of the following: debris on MAF sensor causing an incorrect reading, improper oil level, low or high fuel pressure or Engine Control Temperature (ECT) Sensor reading inaccurate.

There is a factory service bulletin for the following GMC models:

2000-2008 GMC Sonoma
2000-2008 GMC Sierra
2000-2008 GMC Yukon
2000-2008 GMC Yukon XL
2001-2008 GMC Denali
2001-2008 GMC Denali XL
2001-2008 GMC Savana
2002-2008 GMC Envoy
2002-2008 GMC Safari
2002-2008 GMC Jimmy
2000-2008 GMC Factory Service Bulletin OBDII Code P0171

When is the code detected?

– Fuel injection system does not operate properly.
– The amount of mixture ratio compensation is too small. (The mixture ratio is too lean.)

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Excessive Fuel Consumption

P0171 GMC Description

The Engine Control Module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during Open Loop and Closed Loop. During Open Loop, the ECM determines fuel delivery based on sensor signals without heated oxygen sensor (HO2S) input. During Closed Loop, the ECM adds HO2S inputs to calculate the short and long term fuel trim (FT) adjustments. If the HO2S indicates a lean condition, the fuel trim values will be above 0 percent. If the HO2S indicates a rich condition, the FT values will be below 0 percent. The short term FT values change rapidly in response to the HO2S voltage signals. The long term FT makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1.

GMC Technical Service Bulletins for P0171 code