GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC K Sierra - 4WD (CNG) 🢒 DTC 🢒 DTC P0174

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 100 mV to 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 indicates 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 ECM 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.

Test Description

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

  1. This step determines whether the fault is present.

  2. If DTC P0171 and DTC P0174 is 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.

    3.

  3. A vacuum leak causes DTC P0171 and DTC P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the 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.

Step

Action

Values

Yes

No

1

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

--

Go to Step 2

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

2

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

  1. Idle the engine at the normal operating temperature.
  2. The fuel system is in Closed Loop.
  3. Observe the Long Term FT Avg. display for bank 1 or bank 2 with a scan tool.

Is the Long Term FT Avg. less than the specified value indicated?

45% LT

Go to Step 3

Go to Step 4

3
  1. Observe the Freeze Frame and/or the Failure records data for this DTC.
  2. Turn the ignition OFF for 30 seconds.
  3. Start the engine.
  4. 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 and/or the Failure records data.

Does the DTC fail this ignition cycle?

--

Go to Step 4

Go to Intermittent Conditions

4

Are both banks of the engine operating lean?

--

Go to Step 5

Go to Step 6

5

Inspect for the following conditions:

    • Inspect the vacuum hoses for splits, kinks, and proper connection. Refer to Emission Hose Routing Diagram in Engine Controls in the C/K Pickup Service Manual.
    • Inspect the crankcase ventilation valve and system for leaks.
    • Inspect the PCM and sensor grounds for being clean, tight, and in the correct locations.
    • Inspect the air intake system after the MAF sensor for vacuum leaks.
    • Inspect for an engine mechanical condition. Refer to Symptoms - Engine Mechanical in Engine Mechanical in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 7

Go to Fuel System Diagnosis

6

Inspect for the following conditions:

    • Inspect for exhaust leaks, missing or loose exhaust hardware. Refer to Symptoms - Engine Exhaust in Engine Exhaust in the C/K Pickup Service Manual.
    • Inspect that the HO2S is installed securely and the electrical connector is not contacting the exhaust system or the ignition wires.
    • Inspect for an engine mechanical condition. Refer to Symptoms - Engine Mechanical in Engine Mechanical in the C/K Pickup Service Manual.
    • 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 7

Go to Fuel System Diagnosis

7
  1. Use the scan tool in order to clear the DTCs from the following control modules:
    2. • Alternative Fuels ECM
    • PCM
  2. Turn OFF the ignition for 30 seconds.
  3. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 8

Go to Step 2

8

With a scan tool, select Capture Info in order to observe the stored information.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK