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DTC P0171 or P0172 - Without FX3

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 heated oxygen sensor signal voltages. These changes fine tune the engine fuelling. 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.

Conditions for Running the DTC
    •  The vehicle is operating in closed loop.
    •  No faults related to ECT, IAT, MAP, CMP, CKP, TP, EVAP purge, HO2S, HO2S heater, IAC, EGR, injectors, and misfire exist.
    •  The engine coolant temperature is greater than 69.75°C (157.55°F).
    •  The DTC runs continuously when the above conditions are met.

Conditions for Setting the DTC

P0171 (LMT)

The ECM detects that the average of short term fuel trim values is greater than 26% for 250 s to 500 s.

P0171 (LMU)

The ECM detects that the average of short term fuel trim values is greater than 29% for 150 s to 500 s.

P0172 (LMT)

The ECM detects that the average of short term fuel trim values is less than -24% for 100 s to 500 s.

P0172 (LMU)

The ECM detects that the average of short term fuel trim values is less than -25% for 150 s to 500 s.

Action Taken When the DTC Sets

DTC P0171 and P0172 are Type E DTCs.

Conditions for Clearing the DTC

DTC P0171 and P0172 are Type E DTCs.

Diagnostic Aids
    • The system will become lean if a fuel injector is not supplying enough fuel - DTC P0171.
    • A lean condition may be present during high fuel demand due to a fuel pump that does not deliver enough fuel, a plugged fuel filter, or a restricted fuel pipe - DTC P0171.
    • The system will become rich if a fuel injector is supplying too much fuel - DTC P0172.
    • Review the freeze frame/failure records with a scan tool.

Reference Information

Schematic Reference

Engine Controls Schematics : Siemens ECM - LPG with LCP → Siemens ECM - Gas without FX3 or LCP → Delphi ECM - FX3

Connector End View Reference

Component Connector End 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. Engine running, verify that other DTCs are not set.
    2. ⇒If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Ignition ON, verify that the scan tool MAP sensor pressure parameter is within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure .
    3. ⇒If not within the specified range, refer to DTC P0130-P0134, P0137, P0138, P013A, P013B, or P0140 : FX3 → Without FX3 .
  3. Verify that the scan tool HO2S 1 voltage parameter fluctuates within 200-800 mV.
    4. ⇒If not within the specified range, refer to DTC P0130-P0134, P0137, P0138, P013A, P013B, or P0140 : FX3 → Without FX3 .
  4. Engine at normal operating temperature, verify that the scan tool long term fuel trim parameter is between -5 to +5%.
    5. ⇒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 Diagnosis 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.
    • Excessive fuel in the crankcase - P0172.
    • Restricted air filter - P0172.
    • The incorrect operation of the evaporative emission control system - P0172.
  5. If all conditions test normal, test the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical .
  6. Operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that you observed from the freeze frame/failure records data.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

   


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