DTC P0171 Fuel Trim System Lean. GMC Truck Sonoma Pickup

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1999 🢒 Sonoma Pickup - 2WD 🢒 DTC 🢒 DTC P0171 Fuel Trim System Lean

System Description

In order to provide the best possible combination of driveability, fuel economy, and emission control, a Closed Loop air and fuel metering system is used. While in a Closed Loop, the PCM monitors the oxygen sensor signal voltage. The PCM adjusts fuel delivery based on signal voltage. The Long and the Short Term FT values will indicate a change made to the fuel delivery system. The ideal FT values are around 0 percent. Whenever the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in FT values from 0 to + 100 percent. Whenever the exhaust emissions reach an excessive level due to a lean condition, DTC P0171 will set.

Conditions for Running the DTC

•	No active MAP DTC's

•	No active IAT DTC's

•	No active ECT DTC's

•	No active TP DTC's

•	No active Oxygen Sensor DTC's

•	No active Injector Control DTC's

•	No active Misfire DTC's

•	No active KS DTC's

•	No active CKP DTC's

•	No active CMP DTC's

•	No active EVAP DTC's

•	No active VSS DTC's

•	No active IAC DTC's

•	No active PCM Memory DTC's

•	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 850 RPM and 3400 RPM.

•	Vehicle speed is below 82 mph (132 km/h).

•	MAP is more than 26 kPa.

•	BARO is more than 72 kPa.

•	The fuel level is more than 10 percent.

Conditions for Setting the DTC

•	Fuel Trim Index is above 33%.

•	Above conditions present for over 4 seconds.

Action Taken When the DTC Sets

•	The Malfunction Indicator Lamp (MIL) illuminates if the fault is active within the same conditions for 2 out of 80 ignition cycles.

•	The MIL will illuminates after two consecutive ignition cycles in which the diagnostic runs with the fault active.

•	The PCM records the operating conditions at the time the diagnostic fails. The Freeze Frame and Failure Records buffers stores this information.

•	A history DTC is stored.

Conditions for Clearing the MIL/DTC

•	The MIL will turn off after 3 consecutive ignition cycles in which the diagnostic runs without a fault within the freeze frame conditions that the DTC failed.

•	A history DTC will clear after 40 consecutive warm up cycles without a fault.

•	A scan tool can clear the DTCs.

Diagnostic Aids

•	Check for a poor connection at the PCM. Inspect harness connectors for the following conditions:

-	Backed out terminals

-	Improper mating

-	Broken locks

-	Improperly formed or damaged terminals

-	Poor terminal to wire connection

•	Check for a damaged harness. If the harness appears to be OK, observe the O2S 1 display on the scan tool while moving the connectors and wiring harness related to the engine harness. A change in the display will indicate the location of the fault.

•	Check the brake power booster check valve.

An intermittent may be caused by any of the following conditions:

•	A poor connection

•	Rubbed through wire insulation

•	A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

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

This step indicates that visually and physically inspecting any items which can cause a lean condition can may determine the cause of the DTC being set and save diagnosis time.

Step

Action

Values

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, refer to those DTCs before proceeding with this diagnostic.

Install the scan tool.
Idle the engine in closed loop at the normal operating temperature.
With the scan tool monitor the Long Term FT.

Does the Long Term FT read more than the specified value?

33 %

Go to Step 4

Go to Step 3

Turn ON the ignition, with the engine OFF.
Review the Freeze Frame / Failure Records for this DTC and observe the parameters.
Turn OFF the ignition for 15 seconds.
Operate the vehicle within the Freeze Frame / Failure Records conditions and Conditions for Setting the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 4

Go to Diagnostic Aids

Visually and physically inspect the following items:

•	The exhaust system for leaks.

•	The fuel filter for restriction.

•	The air induction system after the MAF sensor for vacuum leaks.

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

•	The throttle body and intake manifold for vacuum leaks.

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

•	The positive crankcase ventilation valve (PCV), spring and O-ring for proper installation.

•	The AIR system for proper operation (if applicable).

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

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

•	If a problem is found, repair as necessary.

Did you find and correct any of the above conditions?

Go to Step 8

Go to Step 5

Turn OFF the ignition.
Turn ON the ignition, with the engine OFF.
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 within the first specified value range.
When the fuel pump stops, the fuel pressure may vary slightly, it should not drop below the second value in 10 minutes.

Did the fuel pressure measure within the first specified value, then hold within the second specified value for 10 minutes?

284-325 kPa (41-47 psi)

279-320 kPa (36-42 psi)

Go to Step 6

Go to Fuel System Diagnosis