P0171
P0171-FUEL SYSTEM 1/1 LEANP0171 - Fuel System 1/1 Lean:
Theory of Operation
The fuel feedback system will maintain a stoichiometric fuel/air mixture, 14.7:1, by modifying the injector pulse width according to the oxygen content of the exhaust gas. The PCM makes short term and long term fuel corrections to maintain stoiciometric fuel/air ratio for best catalytic converter efficiency. Short term fuel correction is based on upstream O2 sensor output and is designed for quick engine response. The long term fuel correction compensated for variations in the engine specifications, sensor tolerances and component aging and is designed to correct rich and lean conditions over a longer period of time.
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
- O2 signal circuit
- O2 return circuit
- O2 sensor heater operation
- O2 sensor
- MAP sensor operation
- ECT sensor operation
- Engine mechanical
- Fuel filter/pressure regulator (HIGH)
- PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding.
DIAGNOSTIC TEST
1. ACTIVE DTC
Diagnose all other trouble codes before continuing.
NOTE: Check for contaminants that may have damaged an O2 Sensor: contaminated fuel, unapproved silicone, oil and coolant.
Ignition on, engine not running.
With a scan tool, read DTCs.
Q: Is the DTC active at this time?
YES: Go To 2
NO: Refer to the INTERMITTENT CONDITION Diagnostic Procedure. Component Tests and General Diagnostics
Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
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.
Choose a conclusion that best matches your fuel pressure reading.
Within Specification
Go To 3
Above Specification
Replace the fuel filter/pressure regulator. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
Below Specification
Go To 12
CAUTION: Stop All Actuations.
3. O2 SENSOR OPERATION
Start the engine.
Allow the engine to reach normal operating temperature.
NOTE:
- If one of the O2 Sensor's Signal or Return circuit is shorted to ground the scan tool will display all O2 Sensor voltage readings low. The O2 Sensor that is shorted to ground will display a voltage reading near or at 0 volts.
- If one of the O2 Sensor Signal or Return circuits are shorted to voltage, the scan tool will display all O2 Sensor voltage readings high.
- After the repairs have been made, verify proper O2 Sensor operation. If all the O2 Sensor voltage readings have not returned to normal, follow the diagnostic procedure for the remaining O2 Sensors.
With a scan tool, monitor all of the O2 Sensor voltage readings.
Q: Is the voltage switching between 2.5 and 3.4 volts for all the O2 Sensors?
YES: Go To 4
NO: Go To 8
4. 1/1 O2 SENSOR HEATER OPERATION
Turn the ignition off.
NOTE: Wait a minimum of 10 minutes to allow the O2 Sensor to cool down before continuing the test. Allow the O2 Sensor voltage to stabilize at 5.0 volts.
Ignition on, engine not running.
With a scan tool, actuate the 1/1 O2 Heater Test.
With the scan tool, monitor 1/1 O2 Sensor voltage for at least 2 minutes.
Q: Does the voltage stay above 4.5 volts?
YES: Replace the 1/1 O2 Sensor. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
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.
Q: Is the scan tool reading within 1 inches of the Vacuum Gauge reading?
YES: Go To 6
NO: Replace the MAP Sensor. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
6. ECT SENSOR OPERATION
NOTE:
- For this test to be valid, the thermostat must be operating correctly.
- 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).
Q: 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 the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
7. ENGINE MECHANICAL
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 inches 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
Q: Are there any engine mechanical problems?
YES: Repair as necessary. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
NO: Go To 11
8. 1/1 O2 SENSOR
Ignition on, engine not running.
Disconnect the 1/1 O2 Sensor harness connector.
With a scan tool, monitor the 1/1 O2 Sensor voltage.
O2 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) O2 Return Upstream circuit in the O2 Sensor harness connector.
NOTE: The voltage should drop from 5.0 volts to 2.5 volts with the jumper wire in place.
Q: Did the O2 Sensor voltage change from 5.0 volts to 2.5 volts with the jumper wire installed?
YES: Replace the O2 Sensor. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
NO: Go To 9
NOTE: Remove the jumper wire before continuing.
9. (K41) O2 SENSOR 1/1 SIGNAL CIRCUIT
With a scan tool, monitor the 1/1 O2 Sensor voltage reading with the jumper wire removed.
NOTE:
- The scan tool will display all O2 Sensor voltage readings approximately 5.0 volts when only one O2 Sensor's Signal circuit is shorted to voltage.
- The scan tool will display one O2 Sensor voltage close to zero and the others will read lower than normal when one O2 Sensor Signal circuit contains excessive resistance.
Q: Is the voltage above 4.8 volts?
YES: Go To 10
NO: Check the (K41) O2 Sensor 1/1 Signal circuit for a short to ground, open, or short to voltage. Inspect the O2 Sensor connector and the PCM harness connector. If OK, replace and program the Powertrain Control Module. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
10. (K902) O2 RETURN UPSTREAM CIRCUIT
Disconnect the 1/1 O2 Sensor harness connector.
Measure the voltage on the (K902) O2 Return Upstream circuit in the O2 Sensor harness connector.
Q: Is the voltage at 2.5 volts?
YES: Go To 11
NO: Check the O2 Return circuit for a short to ground, open, or short to voltage. Inspect the O2 Sensor connector and the PCM harness connector. If OK, replace and program the Powertrain Control Module. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
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.
Q: Were there any problems found?
YES: Repair as necessary. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
NO: Replace and program the Powertrain Control Module. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
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 the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
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.
Q: Is the Fuel Inlet Strainer plugged?
YES: Replace the Fuel Pump Inlet Strainer. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test
NO: Go To 14
14. FUEL PUMP MODULE
If there are no possible causes remaining, view repair.
Repair
Replace the Fuel Pump Module. Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test