GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Savana (CNG) 🢒 DTC 🢒 DTC P0171

Circuit Description

The heated oxygen sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 360°C (600°F) to produce voltage. The HO2S will produce more voltage when the exhaust is rich, and less voltage when the exhaust is lean. The HO2S operating voltage range is between 0.10 volts (100 mV) to 1 volt (1,000 mV).

The Alternative Fuels ECM utilizes a Closed Loop air/fuel metering system when the HO2S is at operating temperature. During normal Closed Loop fuel control operation, the Alternative Fuels ECM will add fuel when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content, the Alternative Fuels ECM will subtract fuel. This addition and subtraction to the air/fuel mixture may be indicated on a scan tool as the Long and Short Term Fuel Trim values. Short Term Fuel Trim values are adjustments to the fuel mixture that occur quickly. Long Term Fuel Trim values are adjustments to the fuel mixture that accumulate over longer periods of time. The Long and Short Term Fuel Trim values are displayed on a scan tool as both percent and counts. Positive percentage values indicate that fuel is being added to the mixture. Negative percentage values indicate that fuel is being subtracted from the mixture.

This DTC is used to indicate a fully rich fuel trim system that cannot compensate for a lean engine exhaust content.

Conditions for Running the DTC
    • The engine is operating on alternative fuel.
        The engine coolant temperature (ECT) is more than 49°C (120°F).
    • The Alternative Fuels ECU is operating in Closed Loop fuel control.

Conditions for Setting the DTC

The Alternative Fuels ECM monitors that the Short Term Fuel Trim is at plus 50 percent or the Long Term Fuel Trim is at plus 45 percent for at least 5 seconds continuously.

Action Taken When the DTC Sets

    • A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
    • The Alternative Fuels ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
    • The Alternative Fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

    • The Last Test Failed status will clear when the DTC runs and passes.
    • The Alternative Fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
    • The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
    • All DTC Information can be cleared with a scan tool.

Diagnostic Aids

Intermittent Condition

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Engine Controls. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

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

  1. The engine may also have a higher than normal idle if a vacuum leak is present.

    An exhaust system leak may be very small and cause the oxygen sensor to sense a false lean condition.

    2.

  2. When testing the circuits for continuity, the resistance should be less than 5 ohms. When testing the circuits for shorts, the resistance should measure infinite with all components disconnected from the circuit.

    3.

  3. This step determines if a base engine concern is causing the lean condition or if the lean condition is only present when operating on CNG.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Alternative Fuels Engine Controls Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2
  1. Capture the DTC information from the Alternative Fuels ECM into the scan tool.
  2. Inspect both the Alternative Fuels ECM and the gasoline Control Module (VCM/PCM or ECM) for other stored DTCs.

Are other DTCs stored?

Go to Step 3

Go to Step 4

3

Perform the other DTC diagnosis before attempting diagnosis of this DTC.

Is the other DTC diagnosis and/or repair complete?

Go to Step 12

--

4

This DTC may have been set by one of the following conditions:

    • A signal wire intermittently open or shorted to ground.
    • HO2S connector water intrusion
    • A lean engine condition such as the following items:
       - Incorrect CNG fuel pressure
       - Vacuum leaks
       - An inaccurate MAF sensor
       - An intake air duct leak between the MAF sensor and the throttle body
       - A PCV system malfunction
       - Exhaust system leaks between the engine and the oxygen sensors
    • Repair any of the above or similar engine conditions as necessary.

Was a condition found and repaired?

Go to Step 12

Go to Step 5

5
  1. Reconnect all disconnected or removed components.
  2. Using a scan tool, clear all DTC information from both the gasoline Control Module (VCM/PCM or ECM) and the Alternative Fuels ECM.
  3. Using the scan tool, communicate with the Alternative Fuels ECM and reset the Fuel Trim values to zero.
  4. Turn OFF the ignition and wait 30 seconds.
  5. Start the engine.
  6. View the captured data Freeze Frame Records for this DTC that was stored in Step 2. Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set.
  7. Operate the vehicle within the Conditions for Running this DTC and the parameters observed when the DTC set.
  8. Observe the Short Term Fuel Trim values.

Do the fuel trim values indicate a lean mixture condition is present or does a fuel trim DTC set?

Go to Step 7

Go to Step 6

6

The fault is not present. Refer to Diagnostic Aids.

Are any DTCs stored that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List

Go to Intermittent Conditions

7

Perform the AF Fuel System diagnosis and return to this step. Refer to Fuel System Diagnosis .

Was a CNG fuel system problem found and repaired?

Go to Step 12

Go to Step 8

8

  1. Inspect the oxygen sensor signal and low circuit for continuity between the Alternative Fuels ECM connector terminals and the oxygen sensor connector terminals.
  2. Inspect these circuits for shorts to other circuits or to ground.

    3. Ensure that the oxygen sensor, gasoline Control module, and the Alternative Fuels ECM connectors are disconnected before testing the circuits.

  3. Repair the wiring as necessary.

Was a wiring repair made?

Go to Step 12

Go to Step 9

9

  1. Reconnect all disconnected or removed components.
  2. Using a scan tool, clear all DTC information from both the gasoline Control Module (VCM/PCM or ECM) and the Alternative Fuels ECM.
  3. Using the scan tool, communicate with the gasoline Control Module and reset the Fuel Trim values to zero.
  4. Turn OFF the ignition.
  5. Remove the 20-amp CNG fuse in order to force a switch-over to gasoline operation.

    6. The FIL may not illuminate due to the removal of the CNG 20-amp fuse.

  6. Start the engine.
  7. View the captured data Freeze Frame Records for this DTC that was stored in Step 2. Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set.
  8. Operate the vehicle within the Conditions for Running the DTC and the parameters observed when the DTC set.
  9. Observe the Short Term Fuel Trim values.

Do the fuel trim values indicate a lean mixture condition is present or does a fuel trim DTC set?

Go to Step 10

Go to Step 11

10

Repair the possible causes of lean engine operation. Refer to the items in Step 4.

Is the repair complete?

Go to Step 12

--

11

Replace the fuel mixer. Refer to Fuel Mixer Replacement .

Is the replacement complete?

Go to Step 12

--

12
  1. Reconnect all disconnected or removed components, connectors, fuses, etc.
  2. Using a scan tool, clear all DTC information from both the gasoline Control Module (VCM/PCM or ECM) and the Alternative Fuels ECM.
  3. Using the scan tool, communicate with the both the gasoline Control Module and the Alternative Fuels ECM in order to reset the Fuel Trim values to zero.
  4. Start the engine.
  5. View the captured data Freeze Frame Records for this DTC that was stored in Step 2. Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set.
  6. Operate the vehicle within the Conditions for Running the DTC and the parameters observed when the DTC set until the scan tool indicates the diagnostic ran.

Does the scan tool indicate the DTC passed?

System OK

Go to Step 2