GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 1996 🢒 Century 🢒 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 around 128, or 0 percent. If the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128, that is, from 0 percent to 100 percent. If you detect a rich condition, the fuel trim values will be below 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 Setting the DTC
    • DTC's P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0401, P0502, P0503, P0506, P0507, P0601, P1406, P1133, and P1441, not set.
    • Intake Air Temperature (IAT) is between -25° C (-13° F) and 115° C (175° F).
    • Engine Coolant temperature is between 60° C (140° F) and 115° C (239° F).
    • Engine speed is between 575 RPM and 3600 RPM.
    • Vehicle speed is below 70 mph (113 km/h).
    • MAP is above 38 kPa.
    • BARO is greater than 72 kPa.
    • EVAP purge is not occurring.
    • Fuel Trim Index is above 165 (29 %).
    • Above conditions present for over 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.
    • Clear the MIL/DTCs with a scan tool.

Diagnostic Aids
    • Fuel pressure - System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm. Refer to Fuel System Diagnosis .
    • MAP sensor - An output that causes the PCM to sense a lower than normal manifold pressure (high vacuum) can cause the system to go lean. Disconnecting the MAP sensor will allow the PCM to substitute a fixed (default) value for the MAP sensor. If the lean condition is gone when the sensor is disconnected, substitute a known good sensor and recheck. 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 causes a lean exhaust and can set DTC P0171.
    • Check for poor 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
       - Poor terminal to wire connection
    • Check for a damaged harness. Inspect wiring harness for damage. If the harness appears to be OK, observe the O2S 1 display on the scan tool while moving connectors and wiring harness related to the engine harness. A change in the display will indicate the location of the fault.
    • Check brake power booster check valve for possible leaks.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

  1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored on the scan tool for later reference.

    2.

  2. Visually/physically checking items which may cause a lean condition may determine the cause of the DTC being set and save diagnosis time.

    3.

  3. A vacuum leak can change the Fuel Trim Index and set DTC P0171. This step checks the intake manifold for vacuum leaks.

    4.

  4. Contaminants in fuel, such as alcohol or water, can create a lean condition setting DTC P0171. Checking for these contaminants could identify the fault.

    5.

  5. If no faults have been found at this point and no additional DTC's were set, refer to Diagnostic Aids for additional checks and information.

DTC P0171-Fuel Trim System Lean

Step

Action

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Are any component related DTC's set?

--

Go to Component DTC tables

Go to Step 3

3
  1. Turn the ignition switch ON, with the engine OFF.
  2. Operate the vehicle until the Loop Status indicates Closed .

Is the Fuel Trim Index below the specified value?

160 2.2L

Go to Step 4

Go to Step 5

4
  1. Turn the ignition switch ON, with the engine OFF.
  2. Review the Freeze Frame data and note the parameters.
  3. Operate the vehicle within the Freeze Frame Conditions and Conditions For Setting The DTC as noted.

Does the Fuel Trim Index go below the specified value while operating under the specified conditions?

160 2.2L

Go to Step 16

Go to Step 5

5

Preliminary checks-visually/physically check the following items:

    • Vacuum hoses for splits, kinks and improper connections.
    • Crankcase ventilation oil/air separator for proper installation.
    • Exhaust system for corrosion, leaks, loose or missing hardware.
    • O2S 1 is installed securely and the pigtail harness is not contacting exhaust manifold or engine.
    • Fuel for excessive water, alcohol, or other contaminants.
    • PCM and sensor grounds are clean, tight and in their proper locations.

Did any of the above checks isolate a condition requiring repair?

--

Go to Step 7

Go to Step 6

6
  1. Disconnect the MAP sensor electrical connector.
  2. Operate the vehicle in Closed Loop while monitoring the Fuel Trim Index.

Is the Fuel Trim Index below the specified value?

160 2.2L

Go to Step 16

Go to Step 9

7
  1. Repair the fault found in the previous step.
  2. Recheck the Fuel Trim Index while operating the engine.

Is the Fuel Trim Index below the specified value?

160 2.2L

Go to Step 8

Go to Step 9

8

Lean condition is not present.

Does a driveability problem exist?

--

Go to Symptoms

Go to Step 16

9

  1. Visually and physically inspect the following items for vacuum leaks:
    2. • Intake manifold.
    • Throttle body.
    • Injector O-rings.
  2. Repair if any leaks are found.

Did any of the above checks isolate a condition requiring a repair?

--

Go to Step 16

Go to Step 10

10

With the engine idling, are the IAC counts above the specified value?

5

Go to Step 11

Go to Step 12

11

Check the fuel for excessive water, alcohol, or other contaminants and correct the contaminated fuel condition if present.

Was the fuel contaminated?

--

Go to Step 16

Go to Step 13

12
  1. Refer to DTC P0506 Idle Speed Low DTC P0507 Idle Speed High to check the IAC valve performance.
  2. If a problem is found, repair as necessary.

Was a repair necessary?

--

Go to Step 16

Go to Step 13

13
  1. Connect a fuel pressure gauge to the fuel system.
  2. Turn the ignition switch OFF for at least 10 seconds.
  3. Turn the ignition switch ON, the fuel pump will run for approximately 2-3 seconds. It may be necessary to cycle the ignition switch ON more than once to obtain maximum fuel pressure.
  4. Note the fuel pressure with the fuel pump running. The pressure should be within the specified value. When the fuel pump stops, the pressure may vary slightly then hold steady.

Is the fuel pressure steady and does the fuel pressure hold?

--

Go to Step 14

Fuel System Diagnosis

14
  1. Start and idle the engine at normal operating temperature.
  2. The fuel pressure noted in the above step should drop by indicated value.

Does the fuel pressure drop by the indicated value?

21-69 kPa (3-10 psi)

Fuel Injector Solenoid Coil Test

Fuel System Diagnosis

15

Replace the MAP sensor.

Is the action complete?

--

Go to Step 16

--

16
  1. Using the scan tool, clear the DTC's.
  2. Start the engine and idle at normal operating temperature.
  3. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic has ran and passed?

--

Go to Step 17

Go to Step 2

17

Check if any additional DTC's are set.

Are any DTC's displayed that have not been diagnosed?

--

Go to applicable DTC table

System OK, Refer to Diagnostic Aids