DTC P0171 Fuel Trim System Lean. Isuzu S/T Isuzu

For 1990-2009 cars only

Makes 🢒 Isuzu 🢒 1997 🢒 S/T Isuzu - 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 fuel trim values will indicate a change made to the fuel delivery system. The ideal fuel trim values are around 128 (0 percent). Whenever the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128 (0 to 100 percent). Whenever a rich condition is detected, the fuel trim values will read below 128 (0 to -100 percent), indicating that the PCM is reducing the amount of fuel. Whenever the exhaust emissions reach an excessive level due to a lean condition, a fuel trim DTC is set.

Conditions for Setting the DTC

•	DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0401, P0404, P0405, P0502, P0503, P0506, P0507, P0601, P1404, 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 750 RPM and 3400 RPM.

•	Vehicle speed is below 70 mph (113 km/h).

•	MAP is above 27 kPa.

•	BARO is greater than 73.8 kPa.

•	EVAP purge is not occurring.

•	Fuel Trim Index is above 160 (29%).

•	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.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.

Visually/physically checking items which may cause a lean condition may determine the cause of the DTC being set and save diagnosis time.

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

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

If no malfunctions have been found at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.

DTC P0171-Fuel Trim System Lean

Step

Action

Value(s)

Yes

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

Go to Step 2

Go to

Powertrain On Board Diagnostic (OBD) System Check

Are any component related DTCs set?

Go to Component DTC tables

Go to Step 3

Turn the ignition switch ON, with the engine running.
Operate the vehicle until the Loop Status indicates Closed .

Is the Fuel Trim Index below the specified value?

160

Go to Step 5

Go to Step 4

Turn the ignition switch ON, with the engine OFF.
Review the Freeze Frame data and note the parameters.
Operate the vehicle within the Freeze Frame conditions and Conditions For Setting The DTC as noted.

Does the Fuel Trim Index go below the specified value while operating under the specified conditions?

160

Go to Step 16

Go to Step 5

Preliminary checks-visually/physically check the following items:

•	Vacuum hoses for splits, kinks and improper connections.

•	Crankcase ventilation oil/air separator for proper installation.

•	Exhaust system for corrosion, leaks, loose or missing hardware.

•	O2S 1 is installed securely and the pigtail harness is not contacting exhaust manifold or engine.

•	Fuel for excessive water, alcohol, or other contaminants.

•	PCM and sensor grounds are clean, tight and in their proper locations.

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

Go to Step 7

Go to Step 6

Disconnect the MAP sensor electrical connector.
Operate the vehicle in Closed Loop while monitoring the Fuel Trim Index.

Is the Fuel Trim Index below the specified value?

160

Go to Step 16

Go to Step 9

Repair the malfunction found in the previous step.
Recheck the Fuel Trim Index while operating the engine.

Is the Fuel Trim Index below the specified value?

160

Go to Step 8

Go to Step 9

Lean condition is not present.

Does a driveability problem exist?

Go to Symptoms

Go to Step 16

Visually and physically inspect the following items for vacuum leaks:

•	Intake manifold.

•	Throttle body.

•	Injector O-rings.

Repair if any leaks that are found.

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

Go to Step 16

Go to Step 10

Turn the ignition switch ON, with the engine idling.

Are the IAC counts above the specified value?

Go to Step 11

Go to Step 12

Check the fuel for excessive water, alcohol, or other contaminants and correct the contaminated fuel condition if present.

Was the fuel contaminated?

Go to Step 16

Go to Step 13

Refer to DTC P0506 Idle Speed Low or DTC P0507 Idle Speed High to check the IAC valve performance.
If a problem is found, repair as necessary.

Was a repair necessary?

Go to Step 16

Go to Step 13

Connect a fuel pressure gauge to the fuel system.
Turn the ignition switch OFF for at least 10 seconds.
Turn the ignition switch ON, the fuel pump will run for approximately 2-3 seconds. It may be necessary to cycle the ignition switch ON more than once to obtain maximum fuel pressure.
Note the fuel pressure with the fuel pump running. The pressure should be within the specified value. When the fuel pump stops, the pressure may vary slightly then hold steady.

Is the fuel pressure steady and does the fuel pressure hold?

Go to Step 14

Go to

Fuel System Diagnosis

Start and idle the engine at normal operating temperature.
The fuel pressure noted in the above step should drop by indicated value.

Does the fuel pressure drop by the indicated value?

21-69 kPa (3-10 psi)

Go to

Fuel Injector Solenoid Coil Test (ECT Within 10C-35C (50F-95F)

Go to Fuel System Diagnosis

Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement .

Is the action complete?

Go to Step 16

Using the scan tool, clear the DTCs.
Start the engine and idle at normal operating temperature.
Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

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

Go to Step 17

Go to Step 2

Check if any additional DTCs are set.

Are any DTCs displayed that have not been diagnosed?

Go to applicable DTC table

System OK

Refer to Diagnostic Aids