DTC P0171 Fuel Trim System Lean Bank 1. GMC Truck Yukon XL

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 2000 🢒 Yukon XL - 4WD 🢒 DTC 🢒 DTC P0171 Fuel Trim System Lean Bank 1

Circuit Description

The powertrain control module (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 (HO2S) 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 the 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 diagnostic trouble code (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 active mass air flow (MAF) DTCs

•	No active manifold absolute pressure (MAP) DTCs

•	No active intake air temperature (IAT) DTCs

•	No active engine coolant temperature (ECT) DTCs

•	No active throttle position (TP) DTCs

•	No active injector DTCs

•	No active misfire DTCs

•	No active knock sensor (KS) DTCs

•	No active crank sensor DTCs

•	No active ignition control DTCs

•	No active exhaust gas recirculation (EGR) DTCs

•	No active secondary air injection(AIR) DTCs

•	No active evaporative emission (EVAP) DTCs

•	No active vehicle speed (VS) DTCs

•	The ECT is between 50°C (122°F) and 115°C (239°F).

•	The barometric pressure is more than 74 kPa.

•	The MAF is between 5 g/s and 90 g/s.

•	The MAP is between 26 kPa and 90 kPa.

•	The IAT is between -20°C (-4°F) and +90°C (+194°F).

•	The engine speed is between 400 RPM and 3,000 RPM.

•	The TP sensor angle is less than 90 percent.

•	The vehicle speed is less than 137 km/h (85 mph).

•	The fuel level is more than 10 percent.

Conditions for Setting the DTC

•	The average long term fuel trim cell values are above a predetermined threshold.

•	All the above conditions are present for 6 seconds.

Action Taken When the DTC Sets

•	The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. If the diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed.

•	The first time the diagnostic fails, the PCM records the operating conditions in Failure Records.

•

The second consecutive ignition cycle that this diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed, the PCM stores this information in Freeze Frame. The PCM copies any data previously stored in Freeze Frame and copies the data into the Failure Records. The PCM then overwrites the Freeze Frame. The only exception to this is if a misfire DTC was already recorded in Freeze Frame. In this case, the misfire data stays in Freeze Frame and the PCM updates the fuel trim data in the Failure Records.

Conditions for Clearing the MIL/DTC

•	The PCM turns the MIL OFF after three consecutive ignition cycles that the diagnostic runs and does not fail within the same conditions that the DTC last failed.

Important: If the last failure was during a non-typical driving condition, the MIL may remain ON longer than the three ignition cycles. Review the Freeze Frame and Failure Records for the last failure conditions.

•	A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures.

•	A last test failed clears when the diagnostic runs and does not fail.

•	Use a scan tool in order to clear the MIL/DTC.

Diagnostic Aids

•	This DTC sets if the engine runs out of fuel.

•	A fuel delivery malfunction causes this DTC to set. Thoroughly inspect all items that cause a lean condition.

•	For an intermittent condition, refer to Symptoms .

Test Description

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

This step determines whether the fault is present.

Using the Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass or a fail. Operate the vehicle within the same Freeze Frame conditions such as RPM, load, vehicle speed, temperature, etc. that you observed. This will isolate when the DTC failed. For an intermittent condition, refer to Symptoms .

If DTC P0174 is also set, this indicates both banks of the engine are operating lean. Inspect the items that would cause both banks to operate lean.

A vacuum leak causes DTCs P0171 and P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.

Step

Action

Value(s)

Yes

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

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Important: If any DTCs are set, except P0171 and P0174, refer to those DTCs before proceeding with this diagnostic.

Install the scan tool.
Idle the engine at the normal operating temperature.
Fuel system in Closed Loop.
Monitor the LT FT Bn 1 display on the Fuel Trim Data List using the scan tool.

Is the LT FT less than the specified value indicated?

23% LT

Go to Step 3

Go to Step 4

Turn ON the ignition, with the engine OFF.
Review the Freeze Frame and Failure Records data for this DTC and observe the parameters.
Turn OFF the ignition for 15 seconds.
Start the engine.
Operate the vehicle within the conditions required for this diagnostic to run, and as close to the conditions recorded in Freeze Frame and Failure Records as possible. Special operating conditions that you need to meet before the PCM will run this diagnostic, where applicable, are listed in Conditions for Running the DTC.
Select the Diagnostic Trouble Code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool.

Does the scan tool indicate that this diagnostic failed this ignition?

Go to Step 4

Go to Diagnostic Aids

Is DTC P0174 also set?

Go to Step 5

Go to Step 6

Visually and physically inspect the following items:

•	Vacuum hoses for splits, kinks, and proper connection -- Refer to Emission Hose Routing Diagram .

•	Crankcase ventilation valve and system for leaks -- Refer to Crankcase Ventilation System Description .

•	Contaminated fuel -- Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

•	PCM and sensor grounds are clean, tight and in the proper locations -- Refer to Ground Distribution Schematics in Wiring Systems.

•	Air induction system after MAF sensor for vacuum leaks

•	Engine mechanical failure. Refer to Engine Mechanical 4.8L, 5.3L, 6.0L.

Did you find any problems in any of the above areas?

Go to Step 7

Go to Fuel System Diagnosis

Visually and physically inspect the following items:

•	Bank 1 exhaust leaks, missing or loose exhaust hardware -- Refer to Exhaust System Inspection in Engine Exhaust.

•	Bank 1 HO2S 1 is installed securely and the electrical connector not contacting exhaust system or ignition wires

•	Engine mechanical failure -- Refer to Engine Mechanical 4.8L, 5.3L, 6.0L.

•	Vacuum leaks that will only affect bank 1, such as the intake manifold, the injector O rings, etc

Was a problem found in any of the above areas?

Go to Step 7

Go to Fuel System Diagnosis

Repair or replace any faulty items found.

Is the action complete?

Go to Step 8

Select the Diagnostic Trouble Code (DTC) option and the Clear DTC Information option using the scan tool.
Idle the engine at the normal operating temperature.
Select the Diagnostic Trouble Code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool.
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text, if applicable.

Does the scan tool indicate that this test ran and passed?

Go to Step 9

Go to Step 2

Select the Capture Info option and the Review Info option using the scan tool.

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

Go to the applicable DTC table

System OK