DTC Troubleshooting: P0171
DTC P0171: Fuel System Too Lean
NOTE:
Before you troubleshoot, review the general troubleshooting information.
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 are 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.
P0010, P0011: VTC system
P0087, P0088, P0191: Fuel rail pressure sensor
P006A, P0101, P0102, P0103: MAF sensor
P0105, P0107, P0108, P2073, P2074: MAP sensor
P0130, P0133, P1172, P2195, P2237, P2238, P2252, P2A00: A/F sensor (Sensor 1)
P0134, P0135: A/F sensor (Sensor 1) heater
P0137, P0138, P0139, P2270: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) heater
P0201, P0202, P0203, P0204: No. 1?No. 4 cylinder injector(s)
P0443: EVAP canister purge valve
P2648, P2649: VTEC system

DTC Description
Confirmed DTC
Pending DTC
P0171 Fuel System Too Lean
    

1.
Problem verification - Idle:

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.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
AF FB (ST FUEL TRIM)
More than 1.33
      

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:

Problem verification - On-board Snapshot
?1.
Review the on-board snapshot and record the value of the parameters below when the AF FB (ST FUEL TRIM) was at its highest point.
ENGINE SPEED
  
VEHICLE SPEED
  
REL TP SENSOR
  
APP SENSOR
  


?2.
Drive the vehicle according to the conditions determined above.
Signal
Current conditions
Values
Unit
ENGINE SPEED
    
VEHICLE SPEED
    
REL TP SENSOR
    
APP SENSOR
    

?3.
Check the parameter(s) below with the HDS while driving.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
AF FB (ST FUEL TRIM)
More than 1.33
      

Do the current condition(s) match the threshold under the on-board snapshot conditions?

     
YES
     
     
     
The AF FB (ST FUEL TRIM) is too high under the conditions above. Go to step 3.
     
NO
     
     
     
The 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 debris in MAF sensor:

Check for debris in MAF sensor
?1.
Turn the vehicle to the OFF (LOCK) mode.

?2.
Remove the MAF sensor/IAT sensor.

?3.
Check for lint, bugs, and other foreign objects in the sensor.

Is there debris in the MAF sensor/IAT sensor?

     
YES
     
     
     
Remove the debris from the MAF sensor/IAT sensor and then use a vacuum cleaner to remove any debris from the air cleaner housing.?
     
NO
     
     
     
4.
Check for disconnected or damaged components:

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)
Intake air duct (C)
Oil cap (D) and dipstick (E)

Are any components disconnected or damaged?

     
YES
     
     
     
Repair as necessary.?
     
NO
     
     
     
Go to step 5.
5.
Check PCV system:

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.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
AF FB (ST FUEL TRIM)
More than 1.33
      

Do the current condition(s) match the threshold?

     
YES
     
     
     
The PCV system is not leaking. Go to step 6.
     
NO
     
     
     
There is a leak in the PCV system. Check the vacuum hoses for leaks and repair as needed.?
6.
Check for Intake manifold vacuum leaks:

Check for Intake manifold vacuum leaks
?1.
Disconnect the intake air duct from the air cleaner housing.

?2.
Connect a large intake manifold adapter 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.?
     
NO
     
     
     
Reconnect any disconnected components, then go to step 7.
7.
Check MAF sensor reading:

Check MAF sensor reading
?1.
Continue to let the engine idle.

?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

Signal
Current conditions
Values
Unit
ECT SENSOR 1
    

?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.
Signal
Current conditions
Values
Unit
ENGINE SPEED
    

?4.
While holding the engine speed at 2,500 rpm for 30 seconds, check the parameter(s) below with the HDS.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
MAF SENSOR
6.5?9.1
g/s
    

Do the current condition(s) match the threshold?

     
YES
     
     
     
The MAF sensor read value is OK. Go to step 8.
     
NO
     
     
     
The MAF sensor read value is incorrect. Inspect for a cracked, damaged, or incorrectly installed air cleaner assembly. If OK, replace the MAF sensor/IAT sensor.?
8.
Check fuel pressure (low pressure side):

Check fuel pressure (low pressure side)
?1.
Check the fuel pressure under the conditions when the AF FB (ST FUEL TRIM) is 1.33 or more.
Signal
Current conditions
Values
Unit
AF FB (ST FUEL TRIM)
    

Is the fuel pressure within a specified range of fuel pressure test?

     
YES
     
     
     
The fuel pressure (low pressure side) is OK. Go to step 9.
     
NO
     
     
     
Check the correct power supply and ground for the fuel pump and if both are OK, replace the fuel tank unit.?
9.
Check fuel pressure (high pressure side):

Check fuel pressure (high pressure side)
?1.
Turn the vehicle to the OFF (LOCK) mode.

?2.
Turn the vehicle to the ON mode.

?3.
Clear the DTC with the HDS.


?4.
Start the engine.

?5.
Hold the engine speed at 3,000 rpm without load (in P or N) until the radiator fan comes on, then let it idle.

?6.
Select the DI FUEL PRESSURE TEST in the INSPECTION MENU with the HDS.

  

Does the HDS indicate NORMAL?

     
YES
     
     
     
The fuel pressure (high pressure side) is OK. Go to step 10.
     
NO
     
     
     
Go to the troubleshooting for DTC P0087 and DTC P0088.?
10.
Replace fuel:

Replace fuel
NOTE: There is an increased likelihood of a fuel related issue if the lean condition 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.
Turn the vehicle to the OFF (LOCK) mode.

?2.
Drain the fuel from the fuel tank, then refill the fuel tank with known-good fuel.

?3.
Operate the vehicle under the same conditions where the AF FB (ST FUEL TRIM) was 1.33 or more.
ENGINE SPEED
  
VEHICLE SPEED
  
REL TP SENSOR
  
APP SENSOR
  

Signal
Current conditions
Values
Unit
ENGINE SPEED
    
VEHICLE SPEED
    
REL TP SENSOR
    
APP SENSOR
    

?4.
Start the engine.

?5.
Check the parameter(s) below with the HDS while driving.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
AF FB (ST FUEL TRIM)
More than 1.33
      

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. Go to step 11.
     
NO
     
     
     
The fuel was causing the high AF FB (ST FUEL TRIM) value.?
11.
Replace A/F sensor (Sensor 1):

Replace A/F sensor (Sensor 1)
?1.
Turn the vehicle to the OFF (LOCK) mode.

?2.
Replace the A/F sensor (Sensor 1) with a known-good sensor.

?3.
Start the engine.

?4.
Operate the vehicle under the same conditions where the AF FB (ST FUEL TRIM) was 1.33 or more.
ENGINE SPEED
  
VEHICLE SPEED
  
REL TP SENSOR
  
APP SENSOR
  

Signal
Current conditions
Values
Unit
ENGINE SPEED
    
VEHICLE SPEED
    
REL TP SENSOR
    
APP SENSOR
    

?5.
Check the parameter(s) below with the HDS while driving.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
AF FB (ST FUEL TRIM)
More than 1.33
      

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. Reinstall the original A/F sensor, then 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).?
12.
Replace injectors:

Replace injectors
?1.
Turn the vehicle to the OFF (LOCK) mode.

?2.
Replace the injectors with known-good injectors.

?3.
Start the engine.

?4.
Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 1.33 or more.
ENGINE SPEED
  
VEHICLE SPEED
  
REL TP SENSOR
  
APP SENSOR
  

Signal
Current conditions
Values
Unit
ENGINE SPEED
    
VEHICLE SPEED
    
REL TP SENSOR
    
APP SENSOR
    

?5.
Check the parameter(s) below with the HDS while driving.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
AF FB (ST FUEL TRIM)
More than 1.33
      

Do the current condition(s) match the threshold under the on-board snapshot conditions?

     
YES
     
     
     
The injectors are not the cause of the high AF FB (ST FUEL TRIM) value. Substitute a known-good PCM, then recheck.?
     
NO
     
     
     
The injectors were causing the high AF FB (ST FUEL TRIM) value. Replace all injectors as a set.?