GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2000 🢒 Eldorado 🢒 Engine 🢒 Engine Controls - 4.6L 🢒 DTC P0171 Fuel Trim System Lean Bank 1

Circuit Description

The PCM controls a closed loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The PCM monitors the heated oxygen sensor 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 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. You can use a scan tool in order to monitor the Short and Long Term Fuel Trim. The ideal fuel trim values are around 0 percent. A positive Fuel Trim value indicates that the PCM is adding fuel in order to compensate for a lean condition. A negative Fuel Trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition. If the PCM detects an excessively Rich or Lean condition, the PCM sets a DTC. The long term fuel trim diagnostic parameter is an average of several of the long term speed load learn cells which the PCM selects based on the engine speed and the engine load.

Conditions for Running the DTC
    •  No TP sensor, MAP sensor, MAF sensor, HO2S, KS sensor, Idle speed, Injector, Misfire, EGR Flow, CMP sensor DTCs are set.
    • EGR Flow test is inactive.
    •  Engine coolant temperature between 20°C (68°F) and 115°C (239°F).
    •  Barometric pressure more than 75 kPa.
    •  Mass air flow between 3-60 grams per second.
    •  Manifold absolute pressure between 15-85 kPa.
    •  Intake air temperature between -20°C (-4°F) and 100°C (212°F).
    •  Engine speed between 500-4000 RPM.
    •  Throttle angle less than 65 percent.
    •  Vehicle speed less than 131 km/h (82 mph).

Conditions for Setting the DTC

The PCM in not able to correct for a lean condition in cylinder bank 1.

Action Taken When the DTC Sets
    •  The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic has been run and failed.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC
    •  The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using the scan tool Clear DTC Information function.

Diagnostic Aids

If the condition is intermittent, refer to Intermittent Conditions in Symptoms.

Test Description

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

  1. Determines whether the fault is present.

  2. Freeze frame data will aid in locating a hard to find malfunction. For example, if the fault only occurs at high speed, this would be determined by monitoring the fuel trim cells. The fuel trim cell for cruise could be above 95 percent, which indicates a lean condition. The freeze frame will also indicate how many miles ago the condition was present.

    3.

  3. If DTC P0174 is also set indicates both banks are operating lean. Inspect items that would cause both banks to operate lean.

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 there are any other DTCs set, diagnose those DTCs first.

  1. Operate the engine until the scan tool displays 85°C (185°F) and the loop status indicates Closed.
  2. Observe the Long Term FT bank 1 and the Short Term FT bank 1 values.

Are either values less than their specified value?

95% LT

98% ST

Go to Step 3

Go to Step 4

3

  1. Review and record the scan tool Freeze Frame data
  2. Clear this DTC.
  3. Operate vehicle within the Freeze Frame conditions.
  4. With the scan tool, monitor the Specific DTC Information for DTC P0171 until the test runs.

Does the scan tool indicate that DTC P0171 failed last test?

--

Go to Step 4

Go to Diagnostic Aids

4

Is DTC P0174 also set?

--

Go to Step 5

Go to Step 6

5
  1. Visually/physically inspect the following items:
    2. • Vacuum hoses for splits, kinks and proper connections.
    • Throttle body, intake manifold and EGR valve for vacuum leaks.
    • Crankcase ventilation valve and/or system for leaks including a loose or missing Oil Fill cap or oil dipstick.
    • Contaminated fuel, refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • PCM and sensor grounds are clean tight and in proper locations.
    • MAF sensor inlet screen for damage, or a restriction.
    • Air induction system after MAF sensor for vacuum leaks.
    • Engine mechanical failure.
  2. If a condition is found, repair as necessary.

Did you find and correct a condition?

--

Go to Step 7

Go to Fuel System Pressure Test

6
  1. Visually/physically inspect the following items:
    2. • Bank 1 exhaust leaks, missing or loose exhaust hardware.
    • HO2S bank 1 sensor 1 is installed securely and electrical connector not contacting exhaust system.
    • Engine mechanical failure.
    • Vacuum leaks that will only affect bank 1 (Intake manifold, injector O-rings, etc.)
  2. If a condition is found, repair as necessary.

Did you find and correct a condition?

--

Go to Step 7

Go to Fuel System Pressure Test

7
  1. Review and record the scan tool Freeze Frame data.
  2. Clear the DTCs.
  3. Operate the vehicle within the parameters listed in the Conditions for Running the DTC.
  4. With the scan tool, monitor the Specific DTC Information for DTC P0171 until the test runs.

Does the scan tool indicate that DTC P0171 passed?

--

System OK

Go to Step 2