GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Aveo 🢒 DTC 🢒 DTC P0171

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 DTC:

DTC P0171 Fuel Trim System Lean

Conditions for Running the DTC
    • DTCs P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0131, P0132, P0337, P0338, P0341, P0342, P0400, P1319, P1402, P1404, P1405, P1671, and P1672 are not set.
    • The engine is operating.
    • The engine coolant temperature (ECT) is more than 80°C (176°F).
    • The manifold absolute pressure (MAP) is less than 70 kPa (10.2 psi).
    • The fuel system is in closed loop.
    • The DTC runs 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 malfunction indicator lamp (MIL) will illuminate after 3 consecutive trips with a fail.
    • The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.

Conditions for Clearing the MIL/DTC
    • The MIL will turn OFF after 4 consecutive ignition cycles in which the diagnostic runs without a fault.
    • DTCs can be cleared by using the scan tool.
    • Disconnect the ECM battery feed for 10 seconds.

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.
    • 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?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are any DTCs other than P0171 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 Short Term FT parameter with a scan tool.

Is the percentage more than the specified value?

30%

Go to Step 5

Go to Diagnostic Aids

5

  1. Operate the engine at idle.
  2. Observe the heated oxygen sensor (HO2S) 1 voltage 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
    • Low fuel pressure--Refer to Fuel System Diagnosis .
    • Lean fuel injectors--Refer to Fuel Injector Balance Test with Special Tool .
    • Fuel contamination

Did you find and correct the condition?

--

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 P0107

8

  1. Turn OFF the engine.
  2. Test for the following conditions:
    3. • The correct installation of the HO2S
    • The electrical connectors and wires are secured and not contacting the exhaust system.
    • A short to ground or high resistance in the signal circuit of the HO2S sensor--Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
    • A high resistance on the low reference circuit of the HO2S Sensor--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 - 1.2L

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.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 11

11

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK