GM Service Manual Online
For 1990-2009 cars only

Circuit Description

To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. While in closed loop, the PCM monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltages. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0%; if the HO2S signals are indicating a lean condition the PCM wil add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, the PCM will set DTC P0171. The PCMs maximum authority to control long term fuel trim allows a range between -25% and +20%. The PCMs maximum authority to control short term fuel trim allows a range between -27% and +27%.The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells are as follows:

Cell

Purge ON

Purge OFF

Idle (Cell 0)

X

--

Decel (Cell 1)

X

--

Normal (Cell 2)

--

--

Accel (Cell 3)

X

--

High Flow (Cell 4)

--

--

Long Term F T average greater than +19%

Short Term F T greater than +4%

The vehicle may have to be operated in all of the above conditions marked by an X, before the PCM determines fuel trim status. The amount of fuel trim deviates from 0% in each cell and the amount that each cell is used directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.

Conditions for Running the DTC

    • No MAF, MAP, TP, ECT, IAT, CKP, CAM sensor, Misfire, Fuel Injector, VSS, EST, HO2S, EGR, or EVAP DTCs are set.
    • Engine coolant temperature between 60°C (140°F) and 110°C (230°F).
    • Intake air temperature is between -18°C (0°F) and 70°C (158°F).
    • Manifold absolute pressure is between 18 kPa and 80 kPa.
    • Throttle angle is steady and less than 90%.
    • Vehicle speed is less than 132 km/h (82 mph).
    • Engine speed is between 600 and 4000 RPM.
    • Baro is greater than 70 kPa.
    • Air flow is between 3 gm/s and 150 gm/s.

Conditions for Setting the DTC

    • Long term fuel trim is at or near maximum authority of +19%.
    • Short term fuel trim is greater than +4%.

Action Taken When the DTC Sets

    • The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecuitive trip in which the diagnostic has been run and failed.
    • If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

Conditions for Clearing the MIL/DTC

    • The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
    • The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using the scan tool.

Diagnostic Aids

Check for the following conditions:

    •  Heated oxygen sensor wiring - The sensor pigtail may be routed incorrectly and contacting the exhaust system.
    •  Poor PCM to engine block grounds. Refer to Power and Grounding Component Views .
    •  Poor connection at the PCM - Inspect harness connectors for the following conditions:
       -  Backed out terminals. Refer to Repairing Connector Terminals .
       -  Improper mating. Refer to Repairing Connector Terminals .
       -  Broken locks. Refer to Connector Repairs .
       -  Improperly formed or damaged terminals. Refer to Repairing Connector Terminals .
       -  Poor terminal to wire connection. Refer to Repairing Connector Terminals .
    • Fuel pressure low - The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0171 may be set. Refer to Fuel System Diagnosis .
    • Fuel injector(s) faulty - Refer to Fuel Injector Balance Test .
    • Vacuum leaks - Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system. Refer to Visual/Physical Inspection in Symptoms .
    • Exhaust leaks - An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust System Inspection .
    • Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace MAF sensor. Refer to Mass Airflow Sensor Replacement
    • Fuel contamination - Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Poor connection at PCM - Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection(s). Refer to Intermittents and Poor Connections Diagnosis Repairing Connector Terminals and Connector Repairs .
    • Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the affected HO2S display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the fault. Refer to Wiring Repairs .

Review the Fail Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table:

  1. DTCs other than P0171 may indicate a condition present which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0171.

  1. If the DTC P0171 test passes while the Fail Records conditions are being duplicated, the lean condition is intermittent. Refer to Diagnostic Aids or Symptoms for additional information on diagnosing intermittent problems.

Step

Action

Value(s)

Yes

No

1

Was the Powertrain OBD System Check performed?

--

Go to Step 2

Go to the Powertrain On Board Diagnostic (OBD) System Check

2

Are any DTCs set other than P0171?

--

Go To Applicable DTCs

Go to Step 3

3

  1. Start and run the vehicle in closed loop.
  2. Observe Long Term F T and Short Term F T displayed on the scan tool.

Are the displayed values greater than the specified value?

+19%

( L.T.)

+4

(S.T.)

Go to Step 5

Go to Step 4

4

  1. Review and record the scan tool Fail Records data.
  2. Clear the DTC P0171 and operate the vehicle to duplicate the Fail Records conditions.
  3. Monitor the scan tool Specific DTC info for DTC P0171 while operating the vehicle to duplicate the Fail Records conditions.
  4. Continue operating the vehicle until the DTC P0171 test runs and note test result.

Does the scan tool indicate DTC P0171 failed this ignition?

--

Go to Step 5

Go to Diagnostic Aids

5

  1. Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections. Refer to Visual/Physical Inspection in Symptoms .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 6

6

  1. Visually and physically inspect the crankcase ventilation valve for proper installation. Refer to Crankcase Ventilation System Inspection .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 7

7

  1. Inspect the throttle body inlet for damage, or MAF Sensor inlet screen for the presence of foreign objects which may alter the airflow sample through the MAF sensor.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 8

8

Start the engine and note the idle quality.

Is a high or unsteady idle being experienced?

--

Go to Step 9

Go to Step 11

9

With the engine idling, observe the Idle Air Control display on the scan tool.

Is the displayed value less than the specified value? (Value is displayed in counts).

5

Go to Step 11

Go to Step 10

10

  1. Visually and physically inspect the throttle body, intake manifold, EGR valve and the EGR feed pipe for vacuum leaks.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 11

11

  1. Test idle air control system operation. Refer to the Idle Air Control (IAC) System Diagnosis .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 12

12

Test the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Was a problem found?

--

Go to Step 23

Go to Step 13

13

  1. Visually and physically inspect the PCM injector grounds, power grounds and sensor grounds to ensure that they are clean, tight, and in their proper locations. Refer to Power and Grounding Component Views .
  2. If a problem is found, repair as necessary. Refer to Repair Procedures in Wiring Repairs .

Was a problem found?

--

Go to Step 23

Go to Step 14

14

  1. Perform the Evaporative Emission (EVAP) Control System Diagnosis. Refer to Evaporative Emission Control System Diagnosis .
  2. If a problem is found, repair as necessary.

Was problem found?

--

Go to Step 23

Go to Step 15

15

  1. Perform the Fuel System Pressure Test. Refer to Fuel System Pressure Test .
  2. If a problem is found, repair as necessary.

Was problem found?

--

Go to Step 23

Go to Step 16

16

  1. Visually and physically inspect the intake manifold, injector O-rings, EGR adapter, EGR valve and the EGR feed pipes for vacuum leaks.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 17

17

  1. Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 18

18

  1. Disconnect the MAF sensor electrical connector.
  2. Operate the vehicle in closed loop while monitoring the S.T Fuel Trim displayed on the scan tool.

Does S.T Fuel Trim decrease to near the specified value?

0%

Go to Step 21

Go to Step 19

19

  1. Test fuel injector operation. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 23

Go to Step 20

20

  1. Visually and physically inspect the HO2S to ensure that it is installed securely and that the HO2S pigtail and wiring harness are not contacting the exhaust or otherwise damaged.
  2. If a problem is found, repair as necessary. Refer to Repair Procedures in Wiring Repairs .

Was a problem found?

--

Go to Step 23

Go to Diagnostic Aids

21

Inspect MAF Sensor screen for foreign objects, debris, or damage.

Was a Problem found?

--

Go to Step 23

Go to Step 22

22

Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement .

Is the action complete?

--

Go to Step 23

--

23

  1. Review and record the scan tool Fail Records data.
  2. Clear the DTC P0171.
  3. Operate the vehicle to duplicate the Fail Records conditions.
  4. Monitor the scan tool Specific DTC info for DTC P0171 while operating the vehicle to duplicate the Fail Records conditions.
  5. Continue operating the vehicle until the DTC P0171 test runs and note test result.

Does the scan tool indicate DTC P0171 failed this ignition?

--

Go to Step 2

System OK