DTC Troubleshooting: P0171, P0174
DTC P0171: Rear Bank (Bank 1) Fuel System Too Lean
DTC P0174: Front Bank (Bank 2) Fuel System Too Lean
NOTE:
Before you troubleshoot, record all freeze data and any on-board snapshot, and 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 and/or P0174, troubleshoot those DTCs first, then recheck for P0171 and/or P0174.
P0101, P0102, P0103: MAF sensor
P0107, P0108, P2073, P2074: MAP sensor
P0133, P0153, P1172, P1174, P2195, P2197, P2237, P2238, P2240, P2241, P2243, P2245, P2247, P2249, P2251, P2252, P2254, P2255, P2A00, P2A03: A/F sensor (Sensor 1)
P0134, P0135, P0154, P0155: A/F sensor (Sensor 1) heater
P0137, P0138, P0139, P0157, P0158, P0159, P2270, P2272: Secondary HO2S (Sensor 2)
P0141, P0161: Secondary HO2S (Sensor 2) heater
P0201, P0202, P0203, P0204, P0205, P0206: Injector
P0400, P0401, P0404, P0406, P2413: EGR system
P0443: EVAP canister purge valve
P2279: Intake air leakage
P2646, P2647, P2648, P2649, P2651, P2652, P2653, P2654, P2656, P2657, P2658, P2659: VTEC system

DTC Description
Confirmed DTC
Pending DTC
Freeze Frame
P0171 Rear Bank (Bank 1) Fuel System Too Lean
      
P0174 Front Bank (Bank 2) Fuel System Too Lean
      

1.
Problem verification-Idle:

Problem verification-Idle
?1.
Start the engine, and let it idle without load (A/T in P or N, M/T in neutral).

?2.
Check the parameter(s) below with the HDS.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
ST FUEL TRIM B1
More than 1.33
      
ST FUEL TRIM B2
More than 1.33
      

Do the current condition(s) match the threshold?

     
YES
     
     
     
The ST FUEL TRIM (B1, B2) is too high at idle. Go to step 3.
     
NO
     
     
     
The ST FUEL TRIM (B1, B2) 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 ST FUEL TRIM (B1, B2) 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
ST FUEL TRIM B1
More than 1.33
      
ST FUEL TRIM B2
More than 1.33
      

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

     
YES
     
     
     
ST FUEL TRIM (B1, B2) is too high under the conditions above. Go to step 3.
     
NO
     
     
     
ST FUEL TRIM (B1, B2) 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.
Press the engine start/stop button to select the OFF 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?

     
YES
     
     
     
Remove the debris from the MAF 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 brake booster:

Check brake booster
?1.
Start the engine, and let it idle.

?2.
Pinch the brake booster vacuum hose (A) connected to the intake manifold.


?3.
Check the parameter(s) below with the HDS with the vacuum hose pinched.
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
ST FUEL TRIM B1
More than 1.33
      
ST FUEL TRIM B2
More than 1.33
      

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.?
6.
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
ST FUEL TRIM B1
More than 1.33
      
ST FUEL TRIM B2
More than 1.33
      

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.?
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)
A/T in P or N, M/T in neutral
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.
A/T
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
MAF SENSOR
9.8?13.8
g/s
    

M/T
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
MAF SENSOR
9.7?13.7
g/s
    

Do the current condition(s) match the threshold?

     
YES
     
     
     
The MAF sensor 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:

Check fuel pressure
?1.
Check the fuel pressure under the conditions when ST FUEL TRIM (B1, B2) is 1.33 or more.
Signal
Current conditions
Values
Unit
ST FUEL TRIM B1
    
ST FUEL TRIM B2
    

Is the fuel pressure between 410?460 kPa (4.18?4.69 kgf/cm2, 59.5?66.7 psi)?

     
YES
     
     
     
The fuel pressure 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.
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.
Press the engine start/stop button to select the OFF 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 ST FUEL TRIM (B1, B2) 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
ST FUEL TRIM B1
More than 1.33
      
ST FUEL TRIM B2
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 ST FUEL TRIM (B1, B2) value. Go to step 10.
     
NO
     
     
     
The fuel was causing the high ST FUEL TRIM (B1, B2) value.?
10.
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.
NOTE: If a large intake manifold adapter cone is not available, leave the intake air duct installed, and remove the brake booster vacuum hose from the brake booster, and inject smoke into the hose.

?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 11.
11.
Replace A/F sensor (Sensor 1)

Replace A/F sensor (Sensor 1)
?1.
Press the engine start/stop button to select the OFF 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 ST FUEL TRIM (B1, B2) 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
ST FUEL TRIM B1
More than 1.33
      
ST FUEL TRIM B2
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 ST FUEL TRIM (B1, B2) value, reinstall the original A/F sensor, then go to step 12.
     
NO
     
     
     
The A/F sensor (Sensor 1) was causing the high ST FUEL TRIM (B1, B2) value. Replace the original A/F sensor (Sensor 1).?
12.
Replace fuel injectors:

Replace fuel injectors
?1.
Press the engine start/stop button to select the OFF mode.

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

?3.
Start the engine.

?4.
Operate the vehicle under the same conditions where ST FUEL TRIM (B1, B2) 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
ST FUEL TRIM B1
More than 1.33
      
ST FUEL TRIM B2
More than 1.33
      

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

     
YES
     
     
     
The fuel injectors are not the cause of the low ST FUEL TRIM (B1, B2) value. Substitute a known-good PCM, then recheck.?
     
NO
     
     
     
The fuel injectors were causing the high ST FUEL TRIM (B1, B2) value. Replace the original fuel injectors.?