DTC Troubleshooting

DTC P0171: Fuel System Too Lean
DTC P0172: Fuel System Too Rich

NOTE:
- Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information.
- If some of the DTCs listed below are stored at the same time as DTC P0171 and/or P0172, troubleshoot those DTCs first, then recheck for P0171 and/or P0172.

P0101, P0102, P0103: Mass airflow (MAF) sensor
P0107, P0108, P1128, P1129: Manifold absolute
pressure (MAP) sensor
P0133, P1157, P2195, P2238, P2252, P2A00: Air fuel
ratio (A/F) sensor (Sensor 1)
P0134, P0135: Air fuel ratio (A/F) sensor (Sensor 1)
heater
P0137, P0138, P0139: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) heater
P0181, P0182, P0183: Fuel temperature sensor
P0191, P0192, P0193: Fuel rail pressure sensor
P1187, P1188: Fuel tank temperature (FTT) sensor
P1192, P1193: Fuel tank pressure (FTP) sensor
P2646, P2647, P2648, P2649: VTEC system

1. Check these parts for vacuum leaks:
- PCV valve
- PCV hose
- Throttle body
- Fuel pressure regulator PI vacuum hose
- Brake booster hose

Are the parts OK?

YES - Go to step 2.

NO - Repair or replace parts with leaks, then go to step 16.

2. Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
3. Check under these conditions:
- Engine coolant temperature (ECT SENSOR 1) above 176 °F (80 °C)
- Transmission in Park or neutral
- All electrical loads off
4. Monitor the ENGINE SPEED in the DATA LIST with the HDS, and hold the engine speed at 2,500 rpm. Once the engine speed is met, hold the engine accelerator pedal steady for more than 10 seconds.

Did the engine speed vary more than 100 rpm from 2,500 rpm?

YES - Repeat step 4.

NO - Go to step 5.

5. While holding the engine speed at 2,500 rpm, check the MAF SENSOR in the DATA LIST with the HDS.

Is there about 5.9-8.5 gm/s?

YES - Go to step 6.

NO - Replace the MAF sensor/IAT sensor. then go to step 16.

6. Check for fuel leaks. Is there a fuel leak?

YES - Repair or replace parts as needed, then go to step 16.

NO - Go to step 7.

7. Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
8. Snap the throttle to an engine speed of 3,000 rpm or more.
9. Check the FUEL RAIL PRESSURE SENSOR in the DATA LIST with the HDS while snapping the throttle.

Does the reading change to 4 kPa (0.58 psi, 0.04 kgf/sq.cm) or more?

YES - Go to step 10.

NO - Replace the fuel rail pressure sensor, then go to step 16.

10. Check the FUEL RAIL PRESSURE SENSOR in the DATA LIST with the HDS at idle.

Is about 204-326 kPa (29.58-47.3 psi, 2.08-3.32 kgf/sq.cm) indicated?

YES - Go to step 17.

NO - Go to step 11.

11. Turn the ignition switch OFF.
12. Remove the fuel rail pressure sensor.
13. Reconnect the fuel rail pressure sensor 3P connector to the sensor.
14. Turn the ignition switch ON (II).
15. Compare the value of the BARO SENSOR and the FUEL RAIL PRESSURE SENSOR in the DATA LIST with the HDS.

Is the reading 22.3 kPa (3.23 psi, 0.23 kgf/sq.cm) or more?

YES - Replace the fuel rail pressure sensor, then go to step 16.

NO - Replace the fuel pressure regulator PI, then go to step 16.

16. Turn the ignition switch ON (II).
17. Reset the PCM with the HDS.
18. Do the PCM idle learn procedure,
19. Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on, then let it idle.
20. Test-drive under these conditions:
- Engine coolant temperature (ECT SENSOR 1) above 158 °F (70 °C)
- Transmission in D position
- Drive at a steady speed between 15-75 mph (20-120 km/h)

NOTE:DTC P0171 and/or P0172 may take up to 80 minutes of test driving to set. Using the HDS, monitor the air fuel feed back average (AF FB AVE). If the AF FB AVE stays within 0.80-1.25, there is no problem at this time.

21. Check for Temporary DTCs or DTCs with the HDS. Is DTC P0171 or P0172 indicated?

YES - Go to step 1.

NO - Troubleshooting is complete. If any other Temporary DTCs or DTCs are indicated, go to the indicated DTCs troubleshooting.