P0171

P0171 - FUEL TRIM SYSTEM LEAN BANK 1
A closed loop air/fuel metering system is used to provide the best combination of driveability, fuel economy, and emission control. In this system. short term fuel control is accomplished by the ECM rapidly responding to the oxygen sensor readings. These changes finely tune engine fueling. The long term fuel trim control is determined by the trends of the short term fuel trim. The ECM uses the long term fuel trim to make adaptation adjustments to the engine fueling. Under normal driving conditions, except power enrichment mode, and extended decelerations, the ECM attempts to maintain a 14.7 to 1 air/fuel ratio. Fuel trim values of 0% are ideal. If the value is above 0%, this indicates the engine is running lean. Therefor, the ECM commands more fuel to the system in attempt to maintain a 14.7 to 1 air/fuel ratio. If the value is below 0%. this indicates the engine is running rich. Therefore, the ECM commands less fuel to the system in attempt to maintain a 14.7 to 1 air/fuel ratio. The ECM stores one long term fuel trim value for each of these ranges: idle/decel and cruise/accel. The idle/decel range is known as the additive fuel adaptation range for which only a small amount of fuel is to be added or removed due to very low engine loads. The cruise/accel range is known as the multiplicative adaptation fuel range for which a multiplying factor of fuel is added or removed due to higher engine loads. DTC P0171 will set if the long term fuel trim value for bank 1 (cylinders 1, 3, 5 which are closest to the front of dash) in the idle/decel or cruise/accel ranges exceed a calibrated rich threshold. This indicates the fuel control system has to add too much fuel (bank 1 is running lean) in order to maintain a 14.7 to 1 air/fuel ratio.

DTC PARAMETERS
DTC P0171 will set if the vehicle is in closed loop and either of the following long fuel trim values for bank 1 have been exceeded:
^ Idle/Decel: 7.7%
OR
^ Cruise/Accel: 24%

DTC P0171 diagnostic runs continuously once the above conditions have been met.

P0171 is a type B DTC

DIAGNOSTIC AIDS

IMPORTANT: Diagnose any ECT, EGR, fuel injector, IAT, heated oxygen sensor. MAF, MAP or TP sensor DTCs first.

IMPORTANT:n LT FT BN 1 CRUISE/ACCEL. (long term fuel trim bank 1) parameter on Scan tool is only updated under moderate to heavy engine loads. As a result, between off idle and around 50 km/h (31 mph) under light throttle, this parameter on Scan tool will not be updated. Use LT FT BN 1 IDLE/DECEL parameter on Scan tool to verify the system is running lean at idle (greater than 7.7%) or LT FT BN 1 CRUISE/ACCEL above 50km/h (31 mph) to verify the system is running lean during cruise (greater than 15%).

IMPORTANT: If DTC P0171 and P0174 are set. a problem may exist in the MAF sensor circuit. If the MAF sensor skews towards a lower voltage, the system will be running lean. Compare sensor voltage to a known good vehicle under the same operating conditions.

Other possible causes if DIC P0171 and P0174 are set:
^ Vacuum leaks:
- Around intake manifold (attached vacuum lines) Between upper and lower intake manifolds
- At the base of the lower intake manifold
- Intake manifold vacuum reservoir
- Power brake booster
^ Fuel pressure low
^ Fuel quality poor or water in fuel
^ EGR valve stuck open
^ ECT skewed towards a higher temperature
^ MAE sensor signal voltage skewed low

Possible causes if only DTC P0171 is set:
^ Vacuum leak on bank 1
^ Fuel injectors clogged
^ Exhaust manifold gasket leak around the HO2S bank 1 sensor 1