GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2000 🢒 Cavalier 🢒 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 (0 percent). If the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128 (0 percent to 100 percent). If you detect a rich condition, the fuel trim values will be below 128 (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, 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 more than 72 kPa.
    • EVAP purge is not occurring.

Conditions for Setting the DTC
    • Fuel Trim Index is above 165 (32 percent).
    • 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.
    • 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 --- 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 Symptoms .
    • 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:
       - Plugged fuel filter
       - Lean exhaust
       - 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
    •  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 malfucntion
    • Check 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/physically checking any items which can cause a lean condition can determine the cause of the DTC being set and save diagnosis time.

    2.

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

    3.

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

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, with the engine OFF.
  2. Install a scan tool.

Are any component DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List/Type

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 17

Go to Step 5

5

  1. Turn OFF the ignition.
  2. Preliminary checks-visually/physically inspect the following items:
    3. • 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 for the following conditions:
       - Plugged fuel filter
       - Excessive water
       - Alcohol contamination
       - Other contaminants
    • PCM and the sensor ground circuits:
       - Clean electrical connections
       - Tight electrical connections
       - Electrical connections in the proper locations

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

--

Go to Step 8

Go to Step 6

6
  1. Start and idle the engine.
  2. With the scan tool, observe the O2S 1 voltage.

Does the O2S 1 voltage read less then the specified value?

44 mV

Go to DTC P0131 O2S Circuit Low Voltage Sensor 1

Go to Step 7

7
  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 16

Go to Step 10

8
  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 9

Go to Step 10

9

The lean condition is not present.

Does a driveability problem exist?

--

Go to Symptoms

Go to Step 17

10

  1. Turn OFF the ignition.
  2. Visually and physically inspect the following items for a vacuum leaks:
    3. • The intake manifold
    • The Throttle body
    • The Injector O-rings
  3. If a problem is found, repair as necessary.

Did you find and correct any condition?

--

Go to Step 17

Go to Step 11

11
  1. Start and idle the engine.
  2. With the scan tool, observe the IAC counts.

Does the IAC counts read above the specified value?

5

Go to Step 12

Go to Step 13

12

  1. Turn OFF the ignition.
  2. Test the fuel for the following conditions:
    3. • Excessive water
    • Alcohol
    • Other contaminants
  3. If a problem is found, repair as necessary.

Was the fuel contaminated?

--

Go to Step 17

Go to Step 14

13
  1. Turn OFF the ignition.
  2. Refer to DTC P0506 Idle Speed Low and DTC P0507 Idle Speed High to test the IAC valve performance.
  3. If a problem is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 14

14
  1. Turn OFF the ignition.
  2. Relieve the Fuel Pressure. Refer to Fuel Pressure Relief .
  3. Install the fuel pressure gage. Refer to Fuel System Diagnosis .
  4. Turn ON the ignition, with the engine OFF.
  5. Bleed the air out of the fuel pressure gage into an approved gasoline container.
  6. Turn OFF the ignition for 10-15 seconds.
  7. Turn ON the ignition, with the engine OFF.

    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?

290-379 kPa (42-55 psi)

Go to Step 15

Go to Fuel System Diagnosis

15
  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

16
  1. Turn OFF the ignition.
  2. Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement .

Is the action complete?

--

Go to Step 17

--

17
  1. Use a scan tool to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running this DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK