Buick Regal: DTC P0171 or P0172

Diagnostic Instructions

• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0171

Fuel Trim System Lean

DTC P0172

Fuel Trim System Rich

Circuit/System Description

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 recenter and restore control to short term fuel trim. The ideal fuel trim values are around 0 %. 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.

Conditions for Running the DTC

• DTCs P0030, P0053, P0101, P0102, P0103, P0106, P0107, P0108, P0131 - P0135, P015A, P015B, P0178, P0179, P0201 - P0204, P0236, P0237, P0238, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0300 - P0304, P0411, P0442, P0455, P0458, P0496, P0506, P0507, P1248, P1249, P124A, P124B, P2147, P2148, P2150, P2151, P2153, P2154, P2156, P2157, P2227, P2228, P2229, P2230, P2269, P2440, P2444, or P2A00 are not set.
• The engine is in Closed Loop status.
• The catalyst monitor diagnostic intrusive test, post 02 diagnostic intrusive test, device control, and evaporative emission leak test are not active.
• The engine coolant temperature (ECT) is between -20 and +130ºC (-4 and +266ºF).
• The intake air temperature (IAT) is between -20 and +150ºC (-4 and +302ºF).
• The manifold absolute pressure (MAP) is between 15 - 255 kPa (2.2 - 37 psi).
• The engine speed is between 400 - 6, 600 RPM.
• The mass air flow (MAF) is between 1 - 512 g/s.
• The barometric pressure (BARO) is greater than 70 kPa (10.2 psi).
• The fuel level is greater than 10 %.
• This diagnostic runs continuously when the above conditions have been met.

Conditions for Setting the DTC

• The average long term fuel trim weighted average value is greater or less than a calibrated value.
• The above condition is present for approximately 3 min after the Conditions for Running the DTC have been met.

Action Taken When the DTC Sets

DTCs P0171 and P0172 are Type B DTCs.

Conditions for Clearing the MIL/DTC

TCs P0171 and P0172 are Type B DTCs.

Diagnostic Aids

• The normal Short Term Fuel Trim and Long Term Fuel Trim parameters should be between +10 and -10 % with 0 % the optimum, with the engine running at operating temperature.
• Any un-metered air into the engine causes this DTC to set. Thoroughly inspect all areas of the engine for vacuum leaks.
• A MAF sensor condition can cause this DTC without setting a MAF DTC. If there is a MAF sensor condition, the MAF sensor parameters will appear to be within range.
• Verify that the air filter is the correct one for this application. Verify that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.
• Certain aftermarket air filters may cause a DTC to set.
• Certain aftermarket air induction systems or modifications to the air induction system may cause a DTC to set.
• Certain aftermarket exhaust system components may cause a DTC to set.

Reference Information

Component View Reference

Powertrain Component Views

Electrical Information Reference

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

1. Ignition ON.

2. Verify no other DTCs are set.

• If any other DTCs are set

Refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.

• Go to next step: If no other DTCs are set

3. Verify DTC P0171 or P0172 is not set.

• If a DTC is set

Refer to Circuit/System Testing.

• Go to next step: If no DTC is set

4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed in the Freeze Frame/Failure Records data.

5. Verify DTC P0171 or P0172 is not set.

• If any of the DTCs set

Refer to Circuit/System Testing.

• Go to next step: If none of the DTCs set

6. All OK.

Circuit/System Testing

1. Ignition ON.

2. Verify the manifold absolute pressure sensor is within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure.

• If not within the specified range

Refer to DTC P0106

• Go to next step: If within the specified range

3. Engine running at the operating temperature.

4. Verify the Long Term Fuel Trim parameter is between -10 % and +10 %.

If greater than +10 %

1. Ignition OFF.
2. Inspect for the conditions listed below:
   • Mass air flow (MAF) sensor signal skewed. If the Short Term Fuel Trim parameter changes greater than 20 % when the MAF sensor is disconnected, refer to DTC P0101.
   • Fuel contamination. Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
   • Missing, restricted, or leaking exhaust components. Refer to Symptoms - Engine Exhaust.
   • Malfunctioning fuel injectors. Refer to Fuel Injector Diagnosis.
   • Split, kinked or improperly connected vacuum hoses.
   • Insufficient fuel in the tank
   • Low fuel pressure. Refer to Fuel System Diagnosis.
   • Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings.
   • Leaks in the air induction system and the air intake ducts
   • Missing air filter element
   • Cracked EVAP canister
   • Evaporative pipes obstructed or leaking
   • Leaks in the crankcase ventilation system. Refer to Crankcase Ventilation System Inspection/Diagnosis.
     • If a condition is found, repair as necessary.
     • If no condition is found, inspect the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical.

• If less than -10 %

1. Ignition OFF.
2. Inspect for the conditions listed below:
   • Mass air flow (MAF) sensor signal skewed. If the Short Term Fuel Trim parameters for both banks changes greater than 20 % when the MAF sensor is disconnected, refer to DTC P0101.
   • Fuel contamination - Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
   • Missing, restricted, or leaking exhaust components - Refer to Symptoms - Engine Exhaust.
   • Malfunctioning fuel injectors - Refer to Fuel Injector Diagnosis
   • Collapsed or restricted air intake duct
   • Excessive fuel pressure - Refer to Fuel System Diagnosis
   • Dirty or restricted air filter
   • Objects blocking the throttle body
   • Improper operation of the evaporative emissions control system
   • Excessive fuel in the crankcase - Change engine oil as necessary and inspect for the following:

G18 High Pressure Fuel Pump=Leaking

Q17 Fuel Injector=Leaking - Refer to Fuel Injector Diagnosis

• If a condition is found, repair as necessary.
• If no condition is found, inspect the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical.
• Go to next step: If between -10 % and +10 %

5. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed in the Freeze Frame/Failure Records data.

6. Verify DTC P0171 or P0172 is not set.

• If any of the DTCs set

Refer to conditions listed above.

• Go to next step: If none of the DTCs set

7. All OK.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

Perform the scan tool Fuel Trim Reset after completing the repair.

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