GM Service Manual Online
For 1990-2009 cars only

Description

This diagnostic monitors the operation of the air/fuel ratio feedback controls. In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control module (PCM) uses a Closed Loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) input and when in Closed Loop adjusts fuel delivery based on the HO2S information. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around 0 percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0 percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0 percent indicate a reduction in fuel. If the PCM detects that the long term fuel trim is more than a predetermined value, this DTC sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P0171 HO2S Fuel Trim System Lean Bank 1
    • DTC P0174 HO2S Fuel Trim System Lean Bank 2

HO2S Indication

Short Term FT Action

Long Term FT Action

Short Term FT Response

Lean Condition

Quick Increase of 1% or More

Slow Increase of 1% or More

Return to 0%

Desired Condition

Stays Near 0%

Stays at Learned +/- Value

Stays Near 0%

Rich Condition

Quick Decrease of -1% or Less

Slow Decrease of -1% or Less

Return to 0%

Conditions for Running the DTC

    • Before the PCM can report that DTC P0171 or P0174 failed, DTC P0442 must run and pass.
    • DTCs P0030, P0050, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0135, P0155, P0300-P0306, P0335, P0339, P0401, P0404, P0442, P0443, P0496, P1128, P1129, P2227, P2228, P2229, P2413, P2414, and P2415 are not set.
    • Engine coolant temperature (ECT) is more than 69°C (156°F).
    • Intake air temperature (IAT) is at least 0°C (32°F).
    • Manifold absolute pressure (MAP) is at least 24 kPa.
    • Engine speed is between 630-4,000 RPM.
    • The engine is operating in Closed Loop.
    • The evaporative emission (EVAP) monitor is not running.
    • DTC P0171 and DTC P0174 run once per drive cycle when the above conditions are met.

Conditions for Setting the DTC

The long term fuel trim of bank 1 or bank 2 is more than 18 percent.

Action Taken When the DTC Sets

    • The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

    • The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Use a scan tool in order to clear the MIL and the DTC.

Diagnostic Aids

Check for any of the following conditions:

    • Inspect for a contaminated oxygen sensor. If contamination is found, determine the cause and correct the condition before replacing the HO2S.
    • Inspect for a damaged wiring harness--Inspect the wiring harness for damage. If the harness appears to be OK, observe the heated oxygen sensor (HO2S) bank 1 sensor 1 display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.
    • Inspect for fuel contamination--Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Test for the correct fuel pressure--The fuel system will be lean if the fuel pressure is too low or the fuel filter is restricted. In order to determine the cause of DTC P0171 or DTC P0174 monitoring the fuel pressure while driving the vehicle at various road speeds may be necessary. Refer to Fuel System Diagnosis .
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers in the diagnostic table.

  1. The Diagnostic System Check - Vehicle prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

  2. This step determines whether the fault is present.

  3. If DTC P0171 and P0174 are set at the same time, this indicates both banks of the engine are operating lean. Inspect the items that would cause both banks to operate lean.

  4. A vacuum leak causes DTC P0171 and P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the positive crankcase ventilation (PCV) valve for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.

  5. This step tests for inadequate fuel delivery from a fuel injector. The fault may be a fuel injector or the fuel injector electrical circuit.

Step

Action

Value(s)

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Important: If any DTCs are set, except P0171 and P0174, refer to those DTCs before proceeding with this diagnostic.

  1. Start the engine.
  2. Idle the engine at the normal operating temperature.
  3. Operate the vehicle until the scan tool indicates Closed Loop.
  4. Observe the Long Term FT parameter for bank 1 or bank 2 with a scan tool.

Does the scan tool display less than the specified value?

18%

Go to Step 3

Go to Step 4

3

  1. Turn ignition ON, with the engine OFF.
  2. Observe the Freeze Frame/Failure Records for this DTC.
  3. Clear the DTCs with a scan tool.
  4. Turn OFF the ignition for 90 seconds.
  5. Start the engine.
  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.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

Are both banks of the engine operating lean?

--

Go to Step 5

Go to Step 6

5

Visually and physically inspect for any of the following conditions:

    • Inspect the vacuum hoses for splits, kinks, and proper connection. Refer to Emission Hose Routing Diagram .
    • Inspect the crankcase ventilation valve and system for leaks.
    • Inspect for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect the engine control grounds for being clean, tight, and in the correct locations.
    • Inspect for an engine mechanical condition. Refer to Symptoms - Engine Mechanical .

Did you find and correct the condition?

--

Go to Step 11

Go to Step 7

6

Visually and physically inspect for any of the following conditions:

    • Inspect for exhaust leaks, missing or loose exhaust hardware. Refer to Exhaust Leakage .
    • Inspect that the heated oxygen sensor (HO2S) is installed securely and the electrical connector is not contacting the exhaust system.
    • Inspect for an engine mechanical condition. Refer to Symptoms - Engine Mechanical .
    • Inspect for vacuum leaks that only affect one bank of the engine. For example, the intake manifold, the injector O-rings.

Did you find and correct the condition?

--

Go to Step 11

Go to Step 7

7

Important: When you have completed the fuel system diagnosis, return to this diagnostic.

Test fuel system for operating lean. Refer to Fuel System Diagnosis .

Did you find and correct the condition?

--

Go to Step 11

Go to Step 8

8

Test the fuel injectors for any of the following conditions:

    • The discharge of fuel is regular and even. Refer to Fuel Injector Balance Test with Tech 2 and Fuel Injector Balance Test with Special Tool .
    • The electrical resistance of the injector coils are all within specifications. Refer to Fuel Injector Solenoid Coil Test .

Did you find and correct a condition?

--

Go to Step 11

Go to Step 9

9

Test for an intermittent and for a poor connection at the appropriate HO2S. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 11

Go to Step 10

10

Replace the HO2S. Refer to the appropriate procedure:

Did you complete the replacement?

--

Go to Step 11

--

11

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 90 seconds.
  3. Start the engine.
  4. Reset the fuel trim system with a scan tool.
  5. 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.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 12

12

In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, operate the vehicle within the conditions for running and setting DTC P0420/0430 and verify that DTC P0420/0430 runs and passes. Refer to DTC P0420 or P0430 .

Does the DTC run and pass?

--

Go to Step 13

Go to DTC P0420 or P0430

13

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK