GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2000 🢒 Sunfire 🢒 DTC 🢒 DTC P0171 Fuel Trim System Lean

System Description

A Closed Loop air/fuel metering system is used to provide the best possible combination of driveability, fuel economy, and emission control. While in a Closed Loop operation, the Powertrain Control Module (PCM) monitors the oxygen sensor signal voltage and adjusts the fuel delivery based on the signal voltage. The long-term fuel trim values and the short-term fuel trim values can indicate any changes to the fuel delivery. Monitor the scan tool for these changes. The ideal fuel trim values are approximately 128, or 0 percent. If the oxygen sensor signal is indicating a lean condition, the PCM will add fuel, resulting in fuel trim values more than 128, that is, from 0 percent to 100 percent. If you detect a rich condition, the fuel trim values will be less than 128, or from 0 percent to -100 percent, indicating that the PCM is reducing the amount of fuel delivered. If the exhaust emissions reach an excessive level due to a lean or a rich condition, a fuel trim DTC is set.

Conditions for Running the DTC
    • DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0201, P0202, P0203, P0204, P0300, P0325, P0335, P0341, P0342, P0446, P0502, P0503, P0506, P0507, P0601, P0602, P1133, and P1441, are not set.
    • The intake air temperature (IAT) is between -25°C (-13°F) and 115°C (175°F).
    • The engine coolant temperature is between 60°C (140°F) and 115°C (239°F).
    • The engine speed is between 550 RPM and 3400 RPM.
    • The vehicle speed is below 82 mph (132 km/h).
    • The MAP is more than 28 kPa.
    • The BARO is more than 72 kPa.
    • The EVAP purge is not occurring.

Conditions for Setting the DTC
    • The fuel trim index is more than 165 (32 percent).
    • The above condition is present for more than 4 seconds.

Action Taken When the DTC Sets
    • The malfunction indicator lamp (MIL) will illuminate if the fault is active within the same conditions for two out of 80 ignition cycles.
        Or
    • The MIL will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.
    • The PCM will record the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers.
    • A history DTC will store.
    • The coolant fan turns ON.

Conditions for Clearing the MIL/DTC
    • The MIL turns OFF after three consecutive ignition cycles during which the diagnostic runs without a fault present under the Freeze Frame conditions that existed when the DTC failed.
    • A history DTC clears after 40 consecutive warm up cycles without a fault.
    • Use a scan tool to clear the DTCs.

Diagnostic Aids

Important: After repairs use the scan tool Fuel Trim Reset function to reset long term fuel trim to 128 (0 percent).

    • Fuel pressure --- The system will be lean if the fuel pressure is too low. It may be necessary to monitor the fuel pressure while driving the vehicle at various road speeds and/or loads to confirm this condition. Refer to Fuel System Diagnosis .
    • The MAP sensor -- An output that causes the PCM to sense a lower than normal manifold pressure, or a high vacuum, can cause the system to go lean. Disconnecting the MAP sensor will allow the PCM to substitute a fixed, or default, value for the MAP sensor. If the lean condition is gone when the sensor is disconnected, substitute a known good sensor and re-check the manifold pressure. Refer to Manifold Absolute Pressure Sensor Output Diagnosis .
    • Fuel contamination -- Water, in even small amounts, near the in-tank fuel pump inlet, can be delivered to the injector. The water can cause the following conditions:
       - A plugged fuel filter
       - Lean exhaust
       - DTC P0171
    • Check for a faulty O2S 1 or MAP sensor connection at the PCM. Inspect the harness connectors for the following conditions:
       - Backed-out terminals
       - Improper mating
       - Broken locks
       - Improperly formed
       - Damaged terminals
       - Faulty terminal-to-wire connection
    •  Inspect the wiring harness for damage. If the harness appears to be OK, observe the O2S 1 display on the scan tool while moving the connectors and the wiring harness that are related to the engine harness. A change in the display will indicate the location of the malfunction
    • Inspect the brake power booster check valve for possible leaks.

Test Description

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

  1. This step indicates that visually and physically inspecting any items which can cause a lean condition can may determine the cause of the DTC being set and save diagnosis time.

Step

Action

Values

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

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

  1. Install the scan tool.
  2. Idle the engine in closed loop at the normal operating temperature.
  3. With the scan tool, monitor the long-term fuel trim (FT).

Does the long-term FT read more than the specified value?

33 %

Go to Step 4

Go to Step 3

3
  1. Turn ON the ignition, with the engine OFF.
  2. Review the Freeze Frame and Failure Records for this DTC and observe the parameters.
  3. Turn OFF the ignition for 15 seconds.
  4. Operate the vehicle within the Freeze Frame and Failure Records conditions and the Conditions for Setting the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 4

Go to Diagnostic Aids

4

Visually and physically inspect the following items:

    • The exhaust system for leaks
    • The fuel filter for restrictions
    • The vacuum hoses for splits, kinks, and proper connections
    • The throttle body and the intake manifold for vacuum leaks
    • The idle air control (IAC), if a high or unsteady idle is being experienced
    • The positive crankcase ventilation valve (PCV), spring and O-ring for proper installation
    • The AIR system for proper operation, if applicable
    • The fuel for excessive water, alcohol, or other contaminates
    • Ensure the PCM and the sensor grounds are clean, tight and in the proper location.
    • If a problem is found, repair the condition as necessary.

Did you find and correct any of the above conditions?

--

Go to Step 8

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Turn ON the ignition, with the engine OFF.
  3. The fuel pump should run for 2 seconds.
  4. The ignition may have to be cycled ON more than once in order to obtain maximum pressure.
  5. Monitor the fuel pressure with the fuel pump running. The fuel pressure should be within the first specified value range.
  6. When the fuel pump stops, the fuel pressure may vary slightly. It should not drop below the second value in 10 minutes.

Did the fuel pressure measure within the first specified value, then hold within the second specified value for 10 minutes?

284-325 kPa (41-47 psi)

279-320 kPa (36-42 psi)

Go to Step 6

Go to Fuel System Diagnosis

6
  1. Start the engine.
  2. Idle the engine at the normal operating temperature.
  3. Observe the fuel pressure gauge.

Does the fuel pressure gauge read within the specified value?

236-277 kPa (34-40 psi)

Go to Step 7

Go to Fuel System Diagnosis

7
  1. Perform the Fuel Injector Balance Test. Refer to Fuel Injector Balance Test .
  2. If a problem is found, repair the fuel injector as necessary.

Did you find and correct the condition?

--

Go to Step 9

Go to Step 11

8
  1. With the scan tool, reset the fuel trim (FT).
  2. Start and operate the vehicle in closed loop.
  3. With the scan tool, observe the long-term FT.

Does the long term FT value decrease below the specified value?

33 %

Go to Step 10

Go to Step 5

9
  1. Replace the fuel injector. Refer to Fuel Injector Replacement .
  2. With the scan tool, reset the fuel trim.
  3. Start and operate the vehicle in closed loop.
  4. With the scan tool, observe the long-term FT.

Does the long-term FT value decrease below the specified value?

33 %

Go to Step 10

Go to Step 5

10

Does a driveability symptom still exists?

--

Go to Symptoms

Go to Step 11

11
  1. Use a scan tool to clear the DTCs.
  2. Operate the vehicle within the Conditions for Setting the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK