GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Oldsmobile 🢒 1999 🢒 Alero 🢒 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 (With EGR)
    • DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0200, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0401, P0404, P0405, P0502, P0503, P0506, P0507, P0601, P0602, P1404, 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 550 RPM and 3400 RPM.
    • Vehicle speed is below 116 km/h (72 mph).
    • MAP is above 26 kPa.
    • BARO is greater than 72 kPa.
    • EVAP purge is not occurring.

Conditions for Running the DTC (Without EGR)
    • DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0200, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0446, P0502, P0503, P0506, P0507, P0601, P0602, P1404, 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 550 RPM and 3400 RPM.
    • Vehicle speed is below 82 mph (132 km/h).
    • MAP is above 28 kPa.
    • BARO is greater than 72 kPa.
    • EVAP purge is not occurring.

Conditions for Setting the DTC
    • Fuel Trim Index is above 165 (32%).
    • 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 eighty 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

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

    • The 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 in order to confirm the fuel pressure. Refer to Fuel System Diagnosis .
    • The 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 no longer present when you disconnect the sensor, substitute a known good sensor and recheck the condition. Refer to Manifold Absolute Pressure Sensor Output Diagnosis .
    • Fuel contamination--Water, in even small amounts, near the in-tank fuel pump inlet, can travel to the fuel injector. The water can cause the following conditions:
      • Plugged fuel filter
      • Lean exhaust
      • DTC P0171
    • Check for a poor O2S 1 or a poor MAP sensor electrical connection at the PCM. Inspect the harness connectors for the following conditions:
       - Backed out terminals
       - Improper mating
       - Broken locks
       - Improperly formed electrical connectors
       - Damaged terminals
       - Poor terminal to wire connection
    • Check for a damaged harness. 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 electrical connectors and the wiring harness related to the engine harness. A change in the display will indicate the location of the malfunction.
    • Check the brake power booster check valve for any possible leaks.

Test Description

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

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

    2.

  2. This step indicates that visually/physically checking any items which can cause a lean condition can 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 malfunction.

    5.

  5. If no malfunction has been present at this point and no additional DTCs were set, refer to Diagnostic Aids for any additional checks and for information.

DTC P0171-Fuel Trim System Lean

Step

Action

Value(s)

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
  1. Turn ON the ignition switch leaving the engine OFF.
  2. Install a scan tool.

Are any component related DTCs set?

--

Go to Component DTC tables

Go to Step 3

3
  1. Start the engine.
  2. Operate the vehicle until the Loop Status indicates Closed.

Is the Fuel Trim Index below the specified value?

29%

Go to Step 4

Go to Step 5

4
  1. Review the Freeze Frame data and record the parameters.
  2. Operate the vehicle within the Freeze Frame conditions and Conditions For Running the DTC as specified.

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

29%

Go to Step 16

Go to Step 5

5

  1. Turn OFF the ignition switch.
  2. Preliminary checks-visually/physically check the following items:
    3. • The EGR vacuum hoses for the following conditions:
       - Splits
       - Kinks
       - Improper connections
    • EGR valve tubes leaking
    • The crankcase ventilation oil/air separator for a proper installation
    • The exhaust system for the following conditions:
       - Corrosion in close proximity of the O2S 1 location
       - Leaks
       - Loose or missing hardware
    • The O2S 1 for the following conditions:
       - Secure installation
       - Electrical connector is not contacting the exhaust manifold or the engine
       - Electrical harness is not contacting the exhaust manifold or the engine
    • Fuel system for the following conditions:
       - Plugged fuel filter
       - Excessive water
       - Alcohol contamination
       - Other contaminants
    • Check the following conditions in the PCM and the sensor ground circuits:
       - Clean electrical connections
       - Tight electrical connections
       - Electrical connections in the proper locations

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

--

Go to Step 7

Go to Step 6

6
  1. Disconnect the MAP sensor electrical connector.
  2. Start the engine.
  3. Operate the vehicle in a Closed Loop operation while monitoring the Fuel Trim Index. (you may have to drive the vehicle to get Closed Loop operations.)

Is the Fuel Trim Index below the specified value?

29%

Go to Step 15

Go to Step 9

7
  1. Repair the malfunction that was found in the previous step.
  2. Start the engine.
  3. Operate the vehicle within the Freeze Frame conditions and Conditions for Running the DTC as specified.
  4. Recheck the Fuel Trim Index.

Is the Fuel Trim Index below the specified value?

29%

Go to Step 8

Go to Step 9

8

The Lean condition is not present.

Does a driveability problem exist?

--

Go to Symptoms

Go to Step 16

9

  1. Turn OFF the ignition switch.
  2. Visually and physically inspect the following items for any vacuum leaks:
    3. • The intake manifold
    • The Throttle body
    • The Injector O-rings
  3. If any leaks are present, repair the leaks.

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

--

Go to Step 16

Go to Step 10

10
  1. Start the engine.
  2. Idle the engine.
  3. Read the IAC counts with the scan tool.

Are the IAC counts above the specified value?

5

Go to Step 11

Go to Step 12

11

  1. Turn OFF the ignition switch.
  2. Check the fuel for the following conditions:
    3. • Excessive water
    • Alcohol
    • Other contaminants
  3. Correct the contaminated fuel condition if present.

Was the fuel contaminated?

--

Go to Step 16

Go to Step 13

12
  1. Turn OFF the ignition switch.
  2. Refer to DTC P0506 Idle Speed Low and DTC P0507 Idle Speed High in order to check the IAC valve performance.
  3. If a problem is present, repair the problem as necessary.

Was a repair necessary?

--

Go to Step 16

Go to Step 13

13
  1. Turn OFF the ignition switch.
  2. Relieve the Fuel Pressure. Refer to Fuel Pressure Relief Procedure .
  3. Install the fuel pressure gage. Refer to Fuel System Diagnosis .
  4. Turn ON the ignition switch.
  5. Bleed the air out of the fuel pressure gage into an approved gasoline container.
  6. Turn OFF the ignition switch for 10-15 seconds.
  7. Turn ON the ignition switch.

    8. Important: The fuel pump will run for approximately 2 seconds. Cycle the ignition switch ON and OFF as necessary in order to achieve the highest possible fuel pressure.

  8. Observe the fuel pressure indicated with the fuel pump running. The fuel pressure should be within the specified value when the fuel pump stops, the fuel pressure may vary slightly then hold steady.

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

284-325 kPa (41-47 psi) with EGR

358-405 kPa (52-58 psi) without EGR

Go to Step 14

Go to Fuel System Diagnosis

14
  1. Start the engine.
  2. Idle the engine at the normal operating temperature.
  3. The fuel pressure that was observed in the above step should drop by the indicated value.

Does the fuel pressure drop by the indicated value?

26-68 kPa (3-10 psi)

Go to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F)

Go to Fuel System Diagnosis

15
  1. Turn OFF the ignition switch.
  2. Replace the MAP sensor. Refer to the Manifold Absolute Pressure Sensor Replacement .

Is the action complete?

--

Go to Step 16

--

16
  1. Clear the DTCs with the scan tool.
  2. Start the engine.
  3. Idle the engine at the normal operating temperature.
  4. Operate the vehicle within the conditions for running 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 to see if any additional DTCs are set.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to applicable DTC table

System OK