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 engine control module (ECM) rapidly responding to the oxygen sensor readings. These changes finely tune the 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 14.7:1 air/fuel ratio. Fuel trim values of 0 percent are ideal. If the value is above 0 percent, this indicates the engine is running lean. Therefore, the ECM commands more fuel to the system in attempt to maintain a 14.7:1 air/fuel ratio. If the value is below 0 percent, this indicates the engine is running rich. Therefore, the ECM commands less fuel to the system in an attempt to maintain a 14.7:1 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, and bank 1 is running lean in order to maintain a 14.7:1 air/fuel ratio.

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:

Important:  Diagnose any engine coolant temperature (ECT), exhaust gas recirculation (EGR), fuel injector, intake air temperature (IAT), heated oxygen sensor, mass air flow (MAF), manifold absolute pressure (MAP), or throttle position (TP) sensor DTCs first.

Important: The LT FT BN 1 CRUISE/ACCEL. (long term fuel trim bank 1) parameter on the 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 the scan tool will not be updated. Use the LT FT BN 1 IDLE/DECEL parameter on the scan tool to verify the system is running lean at idle, greater than 7.7 percent, or the LT FT BN 1 CRUISE/ACCEL above 50 km/h (31 mph) to verify the system is running lean during cruise, greater than 15 percent.

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 the sensor voltage to a known good vehicle under the same operating conditions.

Other possible causes if DTC P0171 and P0174 are set:

Possible causes if only DTC P0171 is set: