GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2004 🢒 Epica 🢒 Engine 🢒 Engine Controls - 2.5L (LBK) 🢒 DTC P0171 or P0174

Circuit Description

The engine control module (ECM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. The fuel trim cells are based on the engine speed and load. If the ECM detects an excessively lean condition, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTCs:

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

Conditions for Running the DTC
    • DTCs P0030, P0031, P0032, P0050, P0051, P0052, P0101, P0102, P0103, P0106, P0107, P0108, P0111, P0112, P0113, P0116, P0117, P0118, P0119, P0121, P0122, P0123, P0125, P0128, P0131, P0132, P0133, P0151, P0152, P0153, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0276, P0277, P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0400, P0401, P0402, P0403, P0404, P0405, P0406, P0444, P0445, P0501, and P1504 are not set.
    • The engine is operating.
    • The engine coolant temperature (ECT) is more 20°C (68°F).
    • The barometric pressure (BARO) is more than 74.5 kPa.
    • The fuel system is in closed loop.
    • The EVAP canister has been purging for more than 240 seconds, then it is disabled for 60 seconds.
    • The DTCs run continuously once the above conditions are met.

Conditions for Setting the DTC

The accumulated value of Long Term and Short Term fuel trim cells are more than 29 percent for 60 seconds.

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
    • The system will become lean if a fuel injector is not supplying enough fuel.
    • A lean condition could be present during high fuel demand due to a fuel pump that does not deliver enough fuel, a plugged fuel filter, or a restricted fuel pipe.
    • Test for fuel contamination. Water or alcohol can effect fuel trim. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions .

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Engine Controls?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are any DTCs other than P0171 or P0174 also set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3
  1. Start the engine.
  2. Observe the TP Sensor percentage parameter with a scan tool.

Is the TP Sensor within the specified range?

2-10 %

Go to Step 4

Go to DTC P0123

4
  1. Start the engine.
  2. Allow the engine to reach the normal operating temperature and enter Closed Loop.
  3. Observe the Long Term FT and Short Term FT parameters with a scan tool.
  4. Add the percentages of the Long Term FT and Short Term FT together.

Is the percentage more than the specified value?

29 %

Go to Step 5

Go to Diagnostic Aids

5

  1. Operate the engine at idle.
  2. Observe the heated oxygen sensor (HO2S) voltage 1 parameter with a scan tool.

Is the voltage within the specified range and fluctuating?

200-800 mV

Go to Step 6

Go to Step 7

6
  1. Turn OFF the engine.
  2. Inspect for the following conditions:
    3. • Splits, kinks, or improper connections at the vacuum hoses--Refer to Emission Hose Routing Diagram .
    • Low fuel pressure--Refer to Fuel System Diagnosis .
    • Lean fuel injectors--Refer to Fuel Injector Balance Test with Special Tool .
    • Fuel contamination. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

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Go to Step 10

Go to Step 9

7
  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.
  4. Compare the MAP sensor parameter on the scan tool to the barometric pressure table. Refer to Altitude Versus Barometric Pressure . The MAP sensor pressure should be within the range specified for your altitude.

Does the MAP parameter indicate the correct barometric pressure?

--

Go to Step 8

Go to DTC P0106

8

  1. Turn OFF the engine.
  2. Inspect for the following conditions:
    3. • The correct installation of the HO2S.
    • The electrical connectors and wires are secured and not contacting the exhaust system.
    • High resistance in the signal circuit or low reference circuit of the H02S Sensor 1. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 10

Go to Fuel System Diagnosis

9
  1. Operate the engine at idle.
  2. Inspect for the following conditions:
    3. • Missing, loose, or leaking exhaust components
    • Vacuum leaks at the intake manifold, throttle body, and injector O-rings
    • Leaks at the air intake ducts
    • Leaking crankcase ventilation system

Did you find and correct the condition?

--

Go to Step 10

Go to Symptoms - Engine Mechanical in Engine Mechanical - 2.5L (LBK)

10
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Reset the fuel trim values 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?

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Go to Step 2

Go to Step 11

11

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