DTC P0171 Fuel Trim System Lean Bank 1. Chevrolet Tahoe 4WD

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 1998 🢒 Tahoe 4WD - RHD 🢒 DTC 🢒 DTC P0171 Fuel Trim System Lean Bank 1

HO2S (5.7L) - Comp Loc Figure
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

In order to provide the best possible combination of driveability, fuel economy, and emission control, the vehicle uses a Closed Loop air and fuel metering system. While in a Closed Loop, the VCM monitors the oxygen sensor signal voltage. The VCM adjusts the fuel delivery based on a signal voltage. The long and short term fuel values, which a scan tool can monitor, indicates a change made to the fuel delivery. Ideal fuel trim values are around 128; if the oxygen sensor signal indicates a lean condition, the VCM adds fuel. This results in fuel trim values above 128. If the oxygen sensor detects a rich condition, the fuel trim values will read below 128. This indicates that the VCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, the VCM sets this DTC.

Conditions for Running the DTC

•	No active O2 sensor DTCs

•	No active TP sensor DTCs

•	No active MAP sensor DTCs

•	No active EGR sensor DTCs

•	No active EVAP system sensor DTCs

•	No active ECT sensor DTCs

•	No active MAF sensor DTCs

•	No active IAT sensor DTCs

•	No active VS sensor DTCs

•	No system voltage DTCs

•	No active misfire DTCs

•	Throttle position is less than 70 %

•	Engine speed is between 575-4,000 RPM

•	The BARO is greater than 70 kPa

•	The ECT is between 75°C-114°C(167°F-237°F)

•	The MAP is between 22-85 kPa

•	The IAT is between -8°C-76°C(46°F-169°F)

•	The MAF is between 3-85 g/s

•	The vehicle speed is less than 85 mph

Conditions for Setting the DTC

The average of the short term fuel trim values over a period of 120 seconds indicates that a lean condition is present.

Action Taken When the DTC Sets

Important: To complete a trip cycle for the fuel trim diagnostic, the fuel trim diagnostic test must enable and run according to the current DTC Will Set When criteria above. A trip is not considered complete until the fuel trim diagnostic has run by the VCM.

When the current DTC initially sets, a software flag indicating an emissions related fault sets for 3 trips. The flag will clear provided the lean condition is no longer present. However, if the VCM detects the conditions for setting the current DTC during a trip while the flag is still set, the VCM turns on the MIL (Malfunction Indicator Lamp). The MIL remains on during all of the subsequent trips until the DTC has been cleared by service or the lean condition has not been detected and the conditions for checking the fuel trim have been met for 3 consecutive trips.

Conditions for Clearing the MIL or DTC

•	The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed.

•

A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.

•	Use a scan tool in order to clear the DTCs.

Diagnostic Aids

If using the diagnostic tables does not isolate the problem, try monitoring the LT fuel trim and fuel trim cell while operating the vehicle under various loads. This may isolate the condition which caused the setting of the current DTC.

A condition that causes the DTC P0300 Engine Misfire Detected may also set the current DTC; conversely an extremely lean condition which sets the current DTC can cause a misfire at idle and the DTC P0300 to set. If the cause of the current DTC cannot be determined and the DTC P0300 is also set, refer to DTC P0300 Engine Misfire Detected .

Test Description

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

Visually and physically check the items that can cause a lean condition in order to determine the cause of the DTC being set.

This step checks for a fault in the EVAP purge system which can cause DTC P0171 to set.

Step

Action

Value(s)

Yes

Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and the Failure Records for reference. This data will be lost when the Clear DTC Information function is used.

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

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Visually and physically check for the following items:

•	The exhaust system for corrosion and loose or missing hardware.

•	The heated oxygen sensors are installed securely.

•	The pigtail harnesses are not contacting the exhaust system.

•	The vacuum hoses for splits, kinks, and proper connections.

•	The throttle body, intake manifold, and EGR valve for vacuum leaks.

•	Idle Air Control (IAC); if a high or unsteady idle is being experienced.

•	The Crankcase Ventilation Valve, spring and O-ring for proper installation.

•	The fuel for excessive water, alcohol, or other contaminates.

•	VCM and sensor grounds are clean, tight and in the proper location.

Is a repair necessary?

Go to Step 8

Go to Step 3

Turn OFF the ignition and make sure the A/C is OFF.
Connect a fuel pressure gauge.
Turn ON the ignition.
The fuel pump should run for 2 seconds.
The ignition may have to be cycled ON more than once in order to obtain maximum pressure.
Monitor the fuel pressure with the fuel pump running. The fuel pressure should be between the specified value.
When the fuel pump stops, the fuel pressure may vary slightly, then should hold steady.

Is pressure between the specified values and does it hold?

415-455 kPa (60-66 psi)

Go to Step 4

Go to Step 11

Start the engine.
Idle the engine until normal operating temperature is reached.

Is the fuel pressure between the specified values?

394-434 kPa (57-63 psi)

Go to Step 5

Go to Step 11

Perform the Injector Balance Test. Refer to Fuel Injector Balance Test .

Was an injector problem found?

Go to Step 9

Go to Step 6

Perform the Canister Purge Solenoid Check. Refer to Evaporative Emission Control System Diagnosis .

Was a problem found?

Go to Step 7

Go to Diagnostic Aids

Repair the EVAP system. Refer to
Connect the scan tool.
Operate the vehicle in a Closed Loop while monitoring LT and ST fuel trim values.

Does the LT fuel trim decrease below the specified value and the ST fuel trim decrease below the second specified value?

158 counts

180 counts

Go to Step 10

Go to Diagnostic Aids

Repair the items found in Step 2.
Operate the vehicle in a Closed Loop while monitoring LT and ST fuel trim values.

Does the LT fuel trim decrease below the specified value and the ST fuel trim decrease below the second specified value?

158 counts

180 counts

Go to Step 10

Go to Step 3

Replace the fuel injector. Refer to Central Sequential Multiport Fuel Injector Replacement in Repair Instructions.
Operate the vehicle in a Closed Loop while monitoring LT and ST fuel trim values.

Does the LT fuel trim decrease below the specified value and the ST fuel trim decrease below the second specified value?

158 counts

180 counts

Go to Step 10

Go to Step 6

The lean condition is not present at this time.

Does a driveability symptom still exist?

Go to Symptoms

Go to Diagnostic Aids

Refer to Fuel System Diagnosis .

Is the action complete?

Go to Step 12

Use the scan tool in order to select the DTC and the Clear DTC Information functions.
Start the Engine.
Allow the engine to idle until normal operating temperature is reached.
Select the DTC and the Specific DTC functions.
Enter the DTC number which was set.
Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

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

Go to Step 13

Go to Step 2

Use the scan tool in order to display the Capture Info and the Review Capture Info functions.

Are there any DTCs displayed that have not been diagnosed?

Go to The Applicable DTC Table

System OK