GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2004 🢒 TrailBlazer - 2WD 🢒 Engine 🢒 Engine Controls - 4.2L 🢒 DTC P0171

Circuit Description

The powertrain control module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during open and Closed Loop. During Open Loop, the PCM determines fuel delivery based on sensor signals, without oxygen sensor (O2S) input. During Closed Loop the PCM adds O2S inputs to calculate Short and Long Term Fuel Trim (FT) fuel delivery adjustments. Short Term FT values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long Term FT values change slower in response to trends in Short Term FT adjustments. Fuel Trim Index is the average of Short Term and Long Term FT and Purge Learn Memory based on engine speed and load. If the PCM detects an excessively lean condition, DTC P0171 sets.

Conditions for Running the DTC
    • DTCs P0013, P0014, P0016, P0068, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0201-P0206, P0220, P0300, P0301-P0304, P0326, P0327, P0341, P0446, P0483, P0496, P0502, P0503, P0507, P0601, P0602, P0604, P0606, P0641, P0651, P1133, P1516, P1680, P1681, P2101, P2120, P2125, P2135, P2138, P2176 are not set.
    • The engine coolant temperature (ECT) is between 60-125°C (140-257°F).
    • The intake air temperature is between -25 and +150°C (-13 and +302°F).
    • The manifold absolute pressure (MAP) is above 6 kPa (0.9 psi).
    • The vehicle speed is less than 132 km/h (82 mph).
    • The engine speed is between 400-5,850 RPM.
    • The barometric pressure is more than 72 kPa (10.3 psi).
    • The fuel level is more than 10 percent.

Conditions for Setting the DTC

The Fuel Trim Index is above 20 percent.

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

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
    • Test 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 .
    • The system will go lean if an injector is not supplying enough fuel.
    • A lean condition could be present during high fuel demand due to a fuel pump that does not pump enough fuel.
    • Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions .

Test Description

The number below refers to the step number on the diagnostic table.

  1. If conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failures may be at fault.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

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

  1. Install a scan tool.
  2. Review the Freeze Frame/Failure Records and record the displayed data for this DTC.
  3. Select Fuel Trim Data parameter.
  4. Start the engine and observe the Fuel Trim Index.

Does the scan tool indicate that the Fuel Trim Index is more than the specified value?

20%

Go to Step 3

Go to Diagnostic Aids

3
  1. Operate the engine at idle.
  2. Observe the O2S parameters with a scan tool.

Does the scan tool indicate that the values are within the specified range and fluctuating?

200-800 mV

Go to Step 4

Go to Step 5

4

  1. Turn OFF the engine.
  2. Visually and physically inspect the following items:
    3. • Vacuum hoses for splits, kinks, and proper connections--Refer to Emission Hose Routing Diagram .
    • If fuel pressure is too low, this DTC may set. Refer to Fuel System Diagnosis .
    • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

--

Go to Step 8

Go to Step 7

5
  1. Turn OFF the engine.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the manifold absolute pressure (MAP) sensor pressure parameter with a scan tool. Refer to Altitude Versus Barometric Pressure .
  4. The MAP sensor pressure should be within the range specified for your altitude.

Does the MAP indicate the correct barometric pressure?

--

Go to Step 6

Go to DTC P0106

6
  1. Turn OFF the engine.
  2. Inspect for the following conditions:
    3. • The oxygen sensor (O2S) for proper installation
    • Ensure electrical connectors and wires are secured and not contacting the exhaust system.
    • A short between the signal circuit and low reference circuit

Did you find and correct the condition?

--

Go to Step 8

Go to Fuel System Diagnosis

7

  1. Operate the engine at idle.
  2. Inspect for the following conditions:
    3. • AIR shut-off valve is stuck open
    • Missing, loose, or leaking exhaust components
    • Vacuum leaks at the intake manifold, throttle body, and injector O-rings
    • The air induction system and air intake ducts for leaks
    • The crankcase ventilation system for leaks

Did you find and correct the condition?

--

Go to Step 8

Go to Symptoms - Engine Mechanical in Engine Mechanical

8

Important: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim.

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition 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/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 9

9

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK