DTC P0171 Fuel Trim System Lean. Chevrolet Cavalier 1995

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 1995 🢒 Cavalier 🢒 DTC 🢒 DTC P0171 Fuel Trim System Lean

System Description

To provide the best possible combination of driveability, fuel economy, and emission control, a Closed Loop air/fuel metering system is used. While in Closed Loop the powertrain control module (PCM) monitors the oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with the scan tool. Ideal fuel trim values are around 128 (0 percent). If the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128 (0-100 percent). If a rich condition is detected, the fuel trim values will be below 128 (0 to -100 percent), indicating that the PCM is reducing the amount of fuel delivered. If exhaust emissions reach an excessive level due to a lean or rich condition, a fuel trim DTC is set.

Conditions for Setting

•	When one or more of the following DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0200, P0300, P0301, P0302, P0303, P0304, P0341, P0342, P0441, P0502, P0503, P0506, P0507, P0600, P1114, P1115, P1133, P1441 are not present.

•	The fuel trim index is above 160 (25 percent).

•	Engine coolant temperature is between 60-115°C (140-239°F).

•	The engine speed is between 750-3,200 RPM.

•	The vehicle speed is below 112 km/h (70 mph).

•	The TP angle is below 80 percent.

•	The MAP is above 38 kPa.

•	The above conditions are present for more than 9 seconds.

Action Taken

Malfunction indicator lamp (MIL) illuminates after two consecutive driving cycles with the fault active.

Conditions for Clearing the MIL/DTC

•	The MIL will turn OFF after passing three consecutive diagnostic tests.

•	A history DTC will clear after 40 consecutive ignition cycles have occurred without a fault.

•	History and current DTCs can be cleared by using a scan tool.

Diagnostic Aids

•	Inspect for poor connections at the PCM. Inspect the harness connectors for backed out terminal, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

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

Test Description

The number below refers to the step number on the diagnostic table.

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 fault.

Step

Action

Values

Yes

Schematic Reference: Engine Controls Schematics

Was the 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 charts

Go to Step 3

Ignition ON, engine ON.
Operate the vehicle while Loop status indicates Closed.
Open the scan tool Engine 1 data list. Observe the Fuel Trim Index. Fuel Trim Index should be below specified value.

Is the Fuel Trim Index below the specified value?

160

Go to Step 4

Go to Step 5

Ignition ON, engine OFF, review the Freeze Frame data and note parameters.
Operate the vehicle within the Freeze Frame conditions noted.
Using a scan tool, monitor the Fuel Trim Index.

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

160

Go to Diagnostic Aids

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 fault 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 still exist?

Go to Symptoms Section

Go to Powertrain On Board Diagnostic (OBD) System Check

Visually and physically inspect the intake manifold throttle body and injector o-rings for vacuum leaks.
Repair if leaks are found.

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

Go to Powertrain On Board Diagnostic (OBD) System Check

Go to Step 10

With the engine idling, observe the idle air control (IAC).

Is the displayed value above the specified value?

Go to Step 11

Go to Step 12

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

Was the fuel contaminated?

Go to Powertrain On Board Diagnostic (OBD) System Check

Go to Step 13

Perform DTC P0506 or P0507 charts to check IAC valve performance.
If a problem is found, repair as necessary.

Did the inspection reveal a condition requiring repair?

Go to Powertrain On Board Diagnostic (OBD) System Check

Go to Step 13

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

Is the pressure steady and does it hold?

284-325 kPa (41-47 psi)

Go to Step 14

Go to Chart A-7

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

Does the fuel pressure drop by the indicated value?

21-69 kPa (3-10 psi)

Go to Step 15

Go to Chart A-7

Remove the crankcase oil/air separator for easy access to fuel injectors.
Perform the Fuel Injector Coil/Balance Test Procedure, Chart C-2A.
Replace any injector found faulty as necessary.

Was an injector fault found?

Go to Powertrain On Board Diagnostic (OBD) System Check

Go to Diagnostic Aids

Replace the MAP sensor.

Is the action complete?

Go to Powertrain On Board Diagnostic (OBD) System Check