DTC Troubleshooting

Control System DTCs

DTC Troubleshooting: P0171

DTC P0171 : Fuel System Too Lean

NOTE:
* Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information How to Troubleshoot the Fuel And Emissions Systems.
* Do not clear the DTC or reset the PCM before beginning this troubleshooting procedure until instructed to clear it. Clearing the DTC or resetting the PCM can delete valuable diagnostic information, even if the on-board snapshot and the freeze data is saved.
* Inspect for aftermarket fuel system and engine components before beginning troubleshooting.
* If any of the DTCs listed below are stored at the same time as DTC P0171, troubleshoot those DTCs first, then recheck for P0171.

P0101, P0102, P0103: MAF sensor

P0106, P0107, P0108: MAP sensor

P0133, P1172, P1157, P2238, P2252, P2A00: A/F sensor (Sensor 1)

P0134, P0135: A/F sensor (Sensor 1) heater

P0137, P0138, P0139, P2270: Secondary HO2S (Sensor 2)

P0201, P0202, P0203, P0204: Fuel injector

P0141: Secondary HO2S (Sensor 2) heater

P0443, P0496: EVAP canister purge valve

P2646, P2647, P2648, P2649: VTEC system

1. Problem verification-Idle:

-1. Start the engine, and let it idle without load (in P or N).

-2. Check the parameter(s) below with the HDS.






Do the current condition(s) match the threshold?
-YES- The AF FB (ST FUEL TRIM) is too high at idle. Go to step 3.
-NO- The AF FB (ST FUEL TRIM) is OK at idle. Go to step 2.

2. Problem verification-On-board Snapshot:

-1. Review the on-board snapshot and record the value of the parameters below when AF FB (ST FUEL TRIM) was at its highest point.






-2. Drive the vehicle according to the conditions determined above.






-3. Check the parameter(s) below with the HDS while driving.






Do the current condition(s) match the threshold under the on-board snapshot conditions?
-YES- AF FB (ST FUEL TRIM) is too high under the conditions above. Go to step 3.
-NO- AF FB (ST FUEL TRIM) is OK under the same conditions as the on-board snapshot. The system is normal at this time.

3. Check for MAF Sensor Debris:

-1. Remove the MAF sensor/IAT sensor. Service and Repair

-2. Check for lint, bugs, and other foreign objects in the sensor.

Is there debris in the MAF sensor?
-YES- Remove the debris from the MAF sensor and then use a vacuum cleaner to remove any debris from the air cleaner housing, then do the repair verification.
-NO- Reinstall the MAF sensor/IAT sensor. Go to step 4.

4. Check for Disconnected or Damaged Components:

-1. Visually inspect each of the components below for disconnected hoses, cracks, or damage:
* Crankcase breather hose (A)
* Air cleaner housing (B)
* Oil cap (C) and dipstick (D)










Are any components disconnected or damaged?
-YES- Repair as necessary, then do the repair verification.
-NO- Go to step 5.

5. Check Brake Booster:

-1. Continue to let the engine idle.

-2. Pinch the brake booster vacuum hose (A) connecting to the intake manifold.






-3. Check the parameter(s) below with the HDS with the vacuum hose pinched.





Do the current condition(s) match the threshold?
-YES- The brake booster is not leaking. Go to step 6.
-NO- There is a leak in the brake booster or hoses. Check the brake booster vacuum hoses for leaks. If no leaks are found, replace the brake booster, then do the repair verification.

6. Check PCV System:

-1. Continue to let the engine idle.

-2. Pinch the PCV vacuum hose (A).






-3. Check the parameter(s) below with the HDS with the vacuum hose pinched.





Do the current condition(s) match the threshold?
-YES- The PCV system is not leaking. Go to step 7.
-NO- There is a leak in the PCV system. Check the vacuum hoses for leaks and repair as needed, then do the repair verification.

7. Check MAF Sensor Reading:

-1. Start the engine and hold the engine speed at 3,000 rpm until the radiator fan comes on.

-2. Make sure the following conditions are met:
* ECT Sensor 1 above 158 °F (70 °C)
* Transmission in P or N
* All engine and electrical loads turned off






-3. Monitor the ENGINE SPEED in the DATA LIST with the HDS, then hold the engine speed at 2,500 rpm with the accelerator pedal held steady.






-4. Check the parameter(s) below with the HDS at 2,500 rpm.






Do the current condition(s) match the threshold?
-YES- The MAF sensor is OK. Go to step 8.
-NO- The MAF sensor is reading incorrectly. Inspect for a cracked, damaged, or incorrectly installed air cleaner assembly. If OK, replace the MAF sensor/IAT sensor Service and Repair, then do the repair verification.

8. Check Fuel Pressure:

-1. Check the fuel pressure under the conditions when AF FB (ST FUEL TRIM) is 1.33 or more.






Is the fuel pressure between 330-380 kPa (3.4-3.9 kgf/cm2, 48-55 psi)?
-YES- The fuel pressure is OK. Go to step 9.
-NO- Replace the fuel tank unit Fuel Tank Unit Removal/Installation, then do the repair verification.

9. Check for Intake Manifold Vacuum Leaks:

-1. Disconnect the intake air duct from the air cleaner housing.

-2. Connect a large intake manifold adaptor cone into the intake air duct from the OTC LeakMaster evaporative emissions system tester kit.

-3. Use the OTC LeakMaster evaporative emissions system tester to direct smoke into the intake manifold.

-4. Inspect each of the areas below for signs of leaks:
* Intake manifold and gaskets
* Throttle body and gasket
* Air intake ducting
* All vacuum and air hoses

Are there any leaks?
-YES- Repair the vacuum leak, then do the repair verification.
-NO- Reconnect any disconnected components, then go to step 10.

10. Replace Fuel Injectors:

-1. Replace the fuel injectors with known-good injectors. Service and Repair

-2. Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 1.33 or more.










-3. Check the parameter(s) below with the HDS while driving.






Do the current condition(s) match the threshold under the on-board snapshot conditions?
-YES- The fuel injectors are not the cause of the high AF FB (ST FUEL TRIM) value. Go to step 11.
-NO- The fuel injectors were causing the high AF FB (ST FUEL TRIM) value. Replace the original injectors Service and Repair, then do the repair verification.

11. Replace A/F Sensor (Sensor 1)

-1. Replace the A/F sensor (Sensor 1) with a known-good sensor. A/F Sensor Removal And Installation

-2. Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 1.33 or more.










-3. Check the parameter(s) below with the HDS while driving.






Do the current condition(s) match the threshold under the on-board snapshot conditions?
-YES- The A/F sensor (Sensor 1) is not the cause of the high AF FB (ST FUEL TRIM) value. Go to step 12.
-NO- The A/F sensor (Sensor 1) was causing the high AF FB (ST FUEL TRIM) value. Replace the original A/F sensor (Sensor 1) A/F Sensor Removal And Installation, then do the repair verification.

12. Replace Fuel:

NOTE: There is an increased likelihood of a fuel related issue if the misfire appeared shortly after refueling. The MIL DIST (MIL Distance) value in the DATA LIST in combination with the fuel level may help you make this determination.

-1. Remove the fuel tank unit and then drain and refill the fuel tank with known-good fuel.

-2. Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 1.33 or more.










-3. Check the parameter(s) below with the HDS while driving.






Do the current condition(s) match the threshold under the on-board snapshot conditions?
-YES- The fuel is not the cause of the high AF FB (ST FUEL TRIM) value. Substitute a known-good PCM Service and Repair, then do the repair verification.
-NO- The fuel was causing the high AF FB (ST FUEL TRIM) value. Do the repair verification.

Repair verificaiton:

-1. Turn the ignition switch to ON (II).

-2. Reset the PCM with the HDS. How to Troubleshoot the Fuel And Emissions Systems

-3. Do the PCM idle learn procedure PCM Idle Learn Procedure.

-4. Start the engine. Hold the engine speed at 3,000 rpm without load (in P or N) until the radiator fan comes on, then let it idle.

-5. Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 1.33 or more.










-6. Check the parameter(s) below with the HDS while driving.






Do the current condition(s) match the threshold under the on-board snapshot conditions?
-YES- AF FB (ST FUEL TRIM) is too high. Go to step 3.
-NO- AF FB (ST FUEL TRIM) is OK. Troubleshooting is complete.