The engine control module (ECM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. The fuel trim cells are based on the engine speed and load.

P0171

The average long term fuel trim cell values are more than 30 percent for 10 seconds.

P0172

The average long term fuel trim cell values are less than -30 percent for 10 seconds.

DTC P0171, and P0172 are Type B DTCs.

DTC P0171, and P0172 are Type B DTCs.

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Engine running, observe the DTC information with a scan tool. Verify no DTCs are set.

⇒	If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

2. Ignition ON, observe the scan tool manifold absolute pressure (MAP) sensor pressure parameter and compare the reading on the scan tool to the barometric pressure table. Refer to Altitude Versus Barometric Pressure .

⇒	If the MAP sensor pressure is not within the specified range for your altitude, refer to DTC P0107 or P0108 .

3. Engine running, observe the scan tool TP sensor percentage parameter. The reading should be less than 10 percent.

⇒	If greater than the specified range, refer to DTC P0122 or P0123 .

4. Engine running, observe the scan tool HO2S sensor voltage parameter. The reading should fluctuate within 200-800 mV.

⇒	If not within specified range, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

5. Engine running at normal operating temperature, observe the scan tool Long Term FT parameter. The reading should be between -5 to +5 percent.

⇒	If not within the specified range, verify none of the following conditions exist:

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

•	Splits, kinks, or improper connections at the vacuum hoses

•	The fuel system is operating lean or rich. Refer to Fuel System Diagnosis .

•	The fuel injectors are operating lean or rich. Refer to Fuel Injector Balance Test with Special Tool .

•	Missing, restricted, or leaking exhaust components--Refer to Symptoms - Engine Exhaust .

•	Vacuum leaks at the intake manifold, throttle body, and injector O-rings P0171

•	Leaking crankcase ventilation system P0171

•	The air intake duct for being collapsed or restricted possibly by a restricted air filter P0172

•	Leaking fuel pressure regulator--Inspect for evidence of fuel in the vacuum line. Refer to Fuel System Diagnosis . P0172

•	Excessive fuel in the crankcase P0172

•	The incorrect operation of the evaporative emissions control system P0172

6. If all conditions test normal, test the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical .
7. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.