P0171
SYMPTOM
P0171-FUEL SYSTEM 1/1 LEAN
WHEN MONITORED
With the engine running in closed loop mode, the ambient/battery temperature above 20 °F and altitude below 8500 ft.
SET CONDITION
If the PCM multiplies short term compensation by long term adaptive and a certain percentage is exceeded for two trips, a freeze frame is stored, the MIL illuminates and a trouble code is stored. Two Trip Fault. Three good trips to turn off the MIL
POSSIBLE CAUSES
- RESTRICTED FUEL SUPPLY LINE
- FUEL PUMP INLET STRAINER PLUGGED
- FUEL PUMP MODULE
- 02 SIGNAL CIRCUIT
- 02 RETURN CIRCUIT
- 02 SENSOR HEATER OPERATION
- 02 SENSOR
- MAP SENSOR OPERATION
- ECT SENSOR OPERATION
- ENGINE MECHANICAL PROBLEM
- FUEL FILTER/PRESSURE REGULATOR (HIGH)
- PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Scan Tool Testing and Procedures
DIAGNOSTIC TEST
1. ACTIVE DTC
Diagnose all other trouble codes before continuing.
Note: Check for contaminants that may have damaged an 02 Sensor: contaminated fuel, unapproved silicone, oil and coolant. Ignition on, engine not running. With a scan tool, read DTCs.
Is the DTC active at this time?
Yes>> Go To 2
No>> Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5. Powertrain Verification Test - Ver 5
2. CHECKING FUEL PRESSURE
WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released. Failure to follow these instructions can result in personal injury or death.
Install a fuel pressure gauge to the fuel rail.
Ignition on, engine not running.
With a scan tool, actuate the ASD Fuel System test and observe the fuel pressure gauge.
Note: Fuel pressure specification is 407 kPa ± 34 kPa (59 psi ± 5 psi).
Turn the ignition off.
CAUTION: Stop All Actuation.
Choose a conclusion that best matches your fuel pressure reading.
Within Specification
Go To 3
Above Specification
Replace the fuel filter/pressure regulator. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
Below Specification
Go To 12
3. 02 SENSOR OPERATION
Start the engine.
Allow the engine to reach normal operating temperature.
Note: If one of the 02 Sensor's Signal or Return circuit is shorted to ground the scan tool will display all 02 Sensor voltage readings low. The 02 Sensor that is shorted to ground will display a voltage reading near or at O volts.
Note: If one of the 02 Sensor Signal or Return circuits are shorted to voltage, the scan tool will display all 02 Sensor voltage readings high.
Note: After the repairs have been made, verify proper 02 Sensor operation. If all the 02 Sensor voltage readings have not returned to normal, follow the diagnostic procedure for the remaining 02 Sensors.
With a scan tool, monitor all of the 02 Sensor voltage readings.
Is the voltage switching between 2.5 and 3.4 volts for all the 02 Sensors?
Yes>> Go To 4
No>> Go To 8
4. 1/1 02 SENSOR HEATER OPERATION
Turn the ignition off.
Note: Wait a minimum of 10 minutes to allow the 02 Sensor to cool down before continuing the test. Allow the 02 Sensor volt age to stabilize at 5.0 volts.
Ignition on, engine not running.
With a scan tool, actuate the 1/1 02 Heater Test.
With the scan tool, monitor 1/1 02 Sensor voltage for at least 2 minutes.
Does the voltage stay above 4.5 volts?
Yes>> Replace the 1/1 02 Sensor. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 5
5. MAP SENSOR OPERATION
Turn the ignition off.
Connect a Vacuum Gauge to a Manifold Vacuum source.
Start the engine.
Allow the engine to idle.
Note: If engine will not idle, maintain a constant RPM above idle.
With the scan tool, read the MAP Sensor vacuum value.
Is the scan tool reading within 1" of the Vacuum Gauge reading?
Yes>> Go To 6
No>> Replace the MAP Sensor. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
6. ECT SENSOR OPERATION
Note: For this test to be valid, the thermostat must be operating correctly.
Note: This test works best if performed on a cold engine (cold soak)
Ignition on, engine not running.
With a scan tool, read the Engine Coolant Temperature (ECT) Sensor value. If the engine was allowed to sit overnight (cold soak), the temperature value should be a sensible value that is somewhere close to the ambient temperature.
Note: If engine coolant temperature is above 82 °C (180 °F), allow the engine to cool until 65 °C (150 °F) is reached.
Start the Engine.
During engine warm-up, monitor the ECT Sensor value. The temperature value change should be a smooth transition from start up to normal operating temperature 82 °C (180 °F). The value should reach at least 82 °C (180 °F).
Did the ECT value increase smoothly and did it reach at least 180 °F (82 °C)?
Yes>> Go To 7
No>> Replace the Engine Coolant Temperature Sensor. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
7. ENGINE MECHANICAL PROBLEM
Turn the ignition off.
Check for any of the following conditions/mechanical problems. AIR INDUCTION SYSTEM - must be free from leaks.
ENGINE VACUUM - must be at least 13 in. in neutral ENGINE VALVE TIMING - must be within specifications
ENGINE COMPRESSION - must be within specifications
ENGINE EXHAUST SYSTEM - must be free of any restrictions or leaks.
ENGINE PCV SYSTEM - must flow freely
TORQUE CONVERTER STALL SPEED - must be within specifications POWER BRAKE BOOSTER - no internal vacuum leaks
FUEL - must be free of contamination
FUEL INJECTOR - plugged or restricted injector; control wire not connected to correct injector
Are there any engine mechanical problems?
Yes>> Repair as necessary. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 11
8. 1/1 02 SENSOR
Ignition on, engine not running.
Disconnect the 1/1 02 Sensor harness connector.
With a scan tool, monitor the 1/1 02 Sensor voltage.
02 Sensor voltage should read 5.0 volts on the scan tool with the connector disconnected.
Connect a jumper wire between the (K41) Signal circuit and the (K902) 02 Upstream Return circuit in the 02 Sensor harness connector.
Note: The voltage should drop from 5.0 volts to 2.5 volts with the jumper wire in place.
Did the 02 Sensor voltage change from 5.0 volts to 2.5 volts with the jumper wire installed?
Yes>> Replace the 02 Sensor. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 9
Note: Remove the jumper wire before continuing.
9. (K41) 02 SENSOR 1/1 SIGNAL CIRCUIT
With a scan tool, monitor the 1/1 02 Sensor voltage reading with the jumper wire removed.
Note: The scan tool will display all 02 Sensor voltage readings approximately 5.0 volts when only one 02 Sensor's Signal circuit is shorted to voltage.
Note: The scan tool will display one 02 Sensor voltage close to zero and the others will read lower than normal when one 02 Sensor Signal circuit contains excessive resistance.
Is the voltage above 4.8 volts?
Yes>> Go To 10
No>> Check the (K41) 02 Sensor 1/1 Signal circuit for a short to ground, open, or short to voltage. Inspect the 02 Sensor connector and the PCM harness connector. If OK, replace and program the Powertrain Control Module per Service Information. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
10. (K902) 02 UPSTREAM RETURN CIRCUIT
Disconnect the 1/1 02 Sensor harness connector.
Measure the voltage on the (K902) 02 Upstream Return circuit in the 02 Sensor harness connector.
Is the voltage at 2.5 volts?
Yes>> Go To 11
No>> Check the 02 Return circuit for a short to ground, open, or short to voltage. Inspect the 02 Sensor connector and the PCM harness connector. If OK, replace and program the Powertrain Control Module per Service Information. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
11. PCM
Note: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.
Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and Ground circuits.
Were there any problems found?
Yes>> Repair as necessary. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Replace and program the Powertrain Control Module per Service Information. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
12. RESTRICTED FUEL SUPPLY LINE
WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released. Failure to follow these instructions can result in personal injury or death.
Raise vehicle on hoist, and disconnect the fuel pressure line at the fuel pump module.
Install special 5/16 fuel line adapter tool # 6539 between disconnected fuel line and the fuel pump module.
Attach a fuel pressure test gauge to the T fitting on tool # 6539. Ignition on, engine not running.
With a scan tool, actuate the ASD Fuel System test and observe the fuel pressure gauge.
Note: Fuel pressure specification is 407 kPa ± 34 kPa (59 psi ± 5 psi).
CAUTION: Stop All Actuations.
Is the fuel pressure within specification?
Yes>> Repair or replace fuel supply line as necessary. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 13
13. FUEL PUMP INLET STRAINER
Turn the ignition off.
WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released.
Remove the Fuel Pump Module and inspect the Fuel Inlet Strainer.
Is the Fuel Inlet Strainer plugged?
Yes>> Replace the Fuel Pump Inlet Strainer. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 14
14. FUEL PUMP MODULE
If there are no possible causes remaining, view repair.
Repair
Replace the Fuel Pump Module. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.