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 percent. If the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0 percent. If a rich condition is detected, the fuel trim values will be below 0 percent, 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 -30 percent and +22 percent. The PCMs maximum authority to control short term fuel trim allows a range between -27 percent and +27 percent. 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)

X

--

Accel (Cell 3)

--

--

High Flow (Cell 4)

--

--

Long term fuel trim average more than +20%

Short term fuel trim more than +3%

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 percent 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

    • MAF, MAP, TP, ECT, IAT, CKP, CAM sensor, Misfire, Fuel Injector, VSS, Ignition, HO2S, EGR, or EVAP DTCs are not set.
    • ECT is between 20°C (68°F) and 110°C (230°F).
    • IAT is between -18°C (0°F) and 70°C (158°F).
    • MAP is between 18 kPa and 80 kPa.
    • Throttle angle is steady and less than 90 percent.
    • Vehicle speed is less than 132 km/h (82 mph).
    • Engine speed is between 600 and 4000 RPM.
    • BARO is more than 70 kPa.
    • Air flow is between 3 gm/s and 150 gm/s.

Conditions for Setting the DTC

    • Long term fuel trim is more than 20 percent.
    • Short term fuel trim is more than 3 percent.

Action Taken When the DTC Sets

    • The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.

Conditions for Clearing the MIL/DTC

    • The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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 a scan tool.

Diagnostic Aids

Inspect 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 Ground Distribution Schematics in Wiring Systems.
    • 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 injectors faulty--Refer to Fuel Injector Balance Test .
    • Vacuum leaks--Inspect 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 in Engine Exhaust.
    • 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 .

Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace MAF sensor. Refer to Mass Airflow Sensor Replacement

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: :  Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

    • Loose terminal connection
       -  Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
       -  Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
    • Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.
    •  Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

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

Values

Yes

No

1

Did you perform the Powertrain On Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to 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 fuel trim and short term fuel trim display on the scan tool data list.

Are the displayed values at or near the specified values?

    • long term fuel trim is the first specified value.
    • Short term fuel trim is the second specified value.

+20%

3%

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

Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections and repair any problem found. Refer to Visual/Physical inspections in Symptoms .

Did you find a problem requiring repair?

--

Go to Step 22

Go to Step 6

6

Visually and physically inspect the crankcase ventilation valve for proper installation and repair any problem found. Refer to Crankcase Ventilation System Inspection .

Did you find a problem requiring repair?

--

Go to Step 22

Go to Step 7

7

  1. Inspect the throttle body inlet screen for damage or for the presence of foreign objects which may partially block the airflow sample through the MAF sensor.
  2. Correct any problem that is found as necessary.

Did you find a condition requiring repair?

--

Go to Step 22

Go to Step 8

8

  1. Start the engine.
  2. Observe 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 IAC display on the scan tool.

Is the displayed value more than the specified value?

5 counts

Go to Step 11

Go to Step 10

10

  1. Visually and physically inspect the following components for vacuum leaks:
  2. • The throttle body
    • The intake manifold
    • The EGR valve
    • The EGR feed pipes
  3. Repair any vacuum leaks as necessary.

Did you find reveal a vacuum leak?

--

Go to Step 22

Go to Step 11

11

  1. Perform Idle Air Control System Diagnosis. Refer to Idle Air Control (IAC) System Diagnosis .
  2. Correct any IAC problem as necessary.

Did find and correct the condition?

--

Go to Step 22

Go to Step 12

12

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

Did you find and correct the condition?

--

Go to Step 22

Go to Step 13

13

  1. Visually and physically inspect the following components to ensure that they are clean, tight, and in their proper locations:
  2. • The PCM injector grounds
    • The PCM grounds
    • The sensor grounds
  3. If a faulty ground condition is present, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 14

14

  1. Disconnect the MAF sensor electrical connector.
  2. Operate the vehicle in Closed Loop while monitoring the short term fuel trim displayed on the scan tool.

Does short term fuel trim value decrease to near the specified value?

0%

Go to Step 21

Go to Step 15

15

Perform the procedure in Fuel System Pressure Test and repair fuel system problem if necessary. Refer to Fuel System Pressure Test .

Did you find and correct the condition?

--

Go to Step 22

Go to Step 16

16

Perform the EVAP Control System Daiagnosis. Refer to Evaporative Emission Control System Diagnosis .

Did you find and correct the condition?

--

Go to Step 22

Go to Step 17

17

  1. Visually and physically inspect the following components for vacuum leaks:
  2. • The intake manifold
    • The injector O-rings
    • The EGR adapter
    • The EGR valve
    • The EGR feed pipes
  3. Repair any problem that is found.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 18

18

Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware and correct any problem found. Refer to Visual/Physical inspection in Symptoms .

Did you find and correct the condition?

--

Go to Step 22

Go to Step 19

19

Perform the Injector Balance Test and correct any problem found. Refer to Fuel Injector Balance Test . Did you find and correct the condition?

--

Go to Step 22

Go to Step 20

20

  1. Ensure that the HO2S is installed securely.
  2. Visually and physically inspect the HO2S to ensure that the pigtail and wiring harness are not contacting the exhaust.
  3. Ensure that the HO2S is not damaged.

Did you find and correct the condition?

--

Go to Step 22

Go to Diagnostic Aids

21

Replace the MAF sensor.

Refer to Mass Airflow Sensor Replacement .

Did you complete the replacement?

--

Go to Step 22

--

22

  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 2

System OK