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 (CL). During Open Loop, the ECM determines fuel delivery based on sensor signals without heated oxygen sensor (HO2S) input. During CL, the ECM adds HO2S inputs and level of purge 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 optimum air/fuel ratio. A block of memory cells contain information arranged in combinations of engine RPM and engine load for a full range of vehicle operating conditions. The long term FT diagnostic is based on an average of cells currently being used. The ECM selects the cells based on the engine speed and load. The FT diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition.

If the ECM detects an excessively lean condition, DTC P0171 sets. If the ECM detects an excessively rich condition, DTC P0172 sets.

3.9L (RPO LGD), VIN code W, E85 compatible engines only--Over estimation of the ethanol content will result in a rich shift of fuel trim values, and under estimation will result in a lean shift of fuel trim values. A fuel trim DTC may set if the learned alcohol content (Fuel Alcohol Content parameter on the scan tool) is different than the measured alcohol content in the vehicle such that fuel trim values exceed failure threshold values.

On vehicles equipped with a vacuum brake booster, rapid brake pedal cycling may imitate a vacuum leak.

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Note: Disregard any transmission symptoms, antilock brake system (ABS) indicators, and traction control system (TCS) indicators until any fuel trim faults are repaired. A fuel trim fault may cause default actions such as harsh shifts and illumination of the ABS/TCS indicators.

Note: The Circuit/System Verification must be performed first or misdiagnosis may result.

P0171

Allow the engine to reach operating temperature. With the engine running, observe the affected Long Term FT parameter with a scan tool. The reading should be between 0-19.5 percent.

1. With the ignition ON and the engine OFF, observe the manifold absolute pressure (MAP) sensor parameter. The MAP sensor pressure should be within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure.

⇒	If the MAP sensor does not indicate the correct barometric pressure, refer to DTC P0106.

2. With the engine idling and the transmission in the Park or Neutral position, observe the manifold absolute pressure (MAP) sensor parameter. The MAP sensor parameter should be between 19-42 kPa.

⇒	If the MAP sensor parameter is not between 19-42 kPa, refer to DTC P0106.

3. With the engine idling, observe the mass air flow (MAF) sensor parameter. The MAF sensor parameter should be between 3-10 g/s at idle.

⇒	If the MAF sensor parameter is not between 3-10 g/s at idle, refer to DTC P0101 or P1101 or DTC P0102 or P0103.

4. Inspect for the following conditions:

•	Vacuum hoses for splits, kinks, and improper connections

•	Insufficient fuel in the tank

•	Low fuel pressure--Refer to Fuel System Diagnosis.

•	Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.

•	Malfunctioning fuel injectors--Refer to Fuel Injector Diagnosis.

•	Missing, loose, or leaking exhaust components from the HO2S forward--Refer to Symptoms - Engine Exhaust.

•	Vacuum leaks at the intake manifold, the throttle body, the brake booster hose, and the injector O-rings

•	The air induction system and the air intake ducts for leaks or for a missing air filter element

•	A cracked evaporative emission (EVAP) canister

•	Evaporative pipes obstructed or leaking

•	The crankcase ventilation system for leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis.

•	The HO2S sensor 1 for improper installation and for electrical wires or connectors that may have contacted the exhaust system

•	The HO2S sensor 1 signal circuit open, shorted to ground, or shorted to the low reference circuit

•	The HO2S sensor 1 low signal circuit for an open circuit or high resistance

•	Malfunctioning engine components--Refer to Symptoms - Engine Mechanical.

Repair or replace as necessary.

P0172

Note: The Circuit/System Verification must be performed first or misdiagnosis may result.

Allow the engine to reach operating temperature. With the engine running, observe the affected Long Term FT parameter with a scan tool. The reading should be between -25 and 0 percent.

1. With the ignition ON and the engine OFF, observe the manifold absolute pressure (MAP) sensor parameter. The MAP sensor pressure should be within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure.

⇒	If the MAP sensor does not indicate the correct barometric pressure, refer to DTC P0106.

2. With the engine idling and the transmission in the Park or Neutral position, observe the manifold absolute pressure (MAP) sensor parameter. The MAP sensor parameter should be between 19-42 kPa.

⇒	If the MAP sensor parameter is not between 19-42 kPa, refer to DTC P0106.

3. With the engine idling, observe the mass air flow (MAF) sensor parameter. The MAF sensor parameter should be within 3-10 g/s at idle.

⇒	If the MAF sensor parameter is not within 3-10 g/s at idle, refer to DTC P0101 or P1101 or DTC P0102 or P0103.

4. Inspect for the following conditions:

•	Vacuum hoses for splits, kinks, and improper connections

•	The air duct for being collapsed or restricted

•	The air filter for being dirty or restricted

•	Objects blocking the throttle body

•	Excessive fuel in the crankcase due to leaking fuel injectors--Refer to Fuel Injector Diagnosis.

•	The evaporative emissions control system for improper operation

•	Excessive fuel pressure--Refer to Fuel System Diagnosis.

•	Malfunctioning fuel injectors--Refer to Fuel Injector Diagnosis.

•	Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.

•	The HO2S sensor 1 for improper installation and for electrical wires or connectors that may have contacted the exhaust system

•	The HO2S sensor 1 signal circuit shorted to voltage

•	Malfunctioning engine components--Refer to Symptoms - Engine Mechanical.

Repair or replace as necessary.

Note: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim values.

1. Install any components or connectors that have been removed or replaced during diagnosis.
2. Perform any adjustment, programming, or setup procedures that are required when a component or module is removed or replaced.
3. Turn ON the ignition, with the engine OFF.

Note: DO NOT clear codes with the engine running. DTCs may reset in the same ignition cycle.

4. Clear the DTCs.
5. Turn OFF the ignition for 60 seconds.
6. Start the engine.
7. Duplicate the Conditions for Running the DTC and use Freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.
8. To verify that the performance of the catalytic converter has not been affected by the condition that set this DTC, perform the Repair Verification for DTC P0420. Refer to DTC P0420.