DTC Troubleshooting: P0172
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.
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 P0172, troubleshoot those DTCs first, then recheck for P0172.
P0010, P0011: VTC system
P0087, P0088: Fuel rail pressure sensor
P0101, P0102, P0103: MAF sensor
P0107, P0108, P2073, P2074: MAP sensor
P0133, P1157, P1172, P2195, 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
P0443: EVAP canister purge valve
P2646, P2647, P2648, P2649: VTEC system

DTC Description
Confirmed DTC
Pending DTC
Freeze Frame
P0172 Fuel System Too Rich
      

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
AF FB (ST FUEL TRIM)
Less than 0.79
      

Do the current condition(s) match the threshold?

     
YES
     
     
     
The AF FB (ST FUEL TRIM) is too low 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 AF FB (ST FUEL TRIM) was at its lowest 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)
Less than 0.79
      

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

     
YES
     
     
     
AF FB (ST FUEL TRIM) is too low 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 fuel pressure (low pressure side):

Check fuel pressure (low pressure side)
?1.
Start the engine, and let it idle.

?2.
Check the fuel pressure under the conditions when AF FB (ST FUEL TRIM) is 0.79 or less.
Signal
Current conditions
Values
Unit
AF FB (ST FUEL TRIM)
    

Is the fuel pressure between 390?440 kPa (4.0?4.5 kgf/cm2, 57?64 psi)?

     
YES
     
     
     
The fuel pressure (low pressure side) is OK. Go to step 4.
     
NO
     
     
     
4.
Check fuel pressure (high pressure side):

Check fuel pressure (high pressure side)
?1.
Turn the ignition switch to LOCK (0), or press the engine start/stop button to select the OFF mode.

?2.
Turn the ignition switch to ON (II), or press the engine start/stop button to select the ON mode.

?3.
Clear the DTC with the HDS.


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

?5.
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 5.
     
NO
     
     
     
Go to the troubleshooting for DTC P0087 and DTC P0088.?
5.
Replace fuel:

Replace fuel
NOTE: There would be an increased likelihood of a fuel related issue if the righ 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 ignition switch to LOCK (0), or 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.
Start the engine, and let it idle.

?4.
Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 0.79 or less.
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)
Less than 0.79
      

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

     
YES
     
     
     
The fuel is not the cause of the low AF FB (ST FUEL TRIM) value. Go to step 6.
     
NO
     
     
     
The fuel was causing the low AF FB (ST FUEL TRIM) value.?
6.
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
6.5?9.1
g/s
    

M/T
Signal
Threshold
Current conditions
Values
Unit
Values
Unit
MAF SENSOR
5.5?7.7
g/s
    

Do the current condition(s) match the threshold?

     
YES
     
     
     
The MAF sensor is OK. Go to step 7.
     
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.?
7.
Check engine oil:

Check engine oil
?1.
Continue to let the engine idle.

?2.
Gently 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)
Less than 0.79
      

Do the current condition(s) match the threshold?

     
YES
     
     
     
The engine oil is OK. Go to step 8.
     
NO
     
     
     
The engine oil is contaminated. Determin the cause of the contaminated oil (faulty injectors, ect.), then replace the engine oil and oil filter.?
8.
Replace A/F sensor (Sensor 1)

Replace A/F sensor (Sensor 1)
?1.
Turn the ignition switch to LOCK (0), or 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, and let it idle.

?4.
Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 0.79 or less.
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)
Less than 0.79
      

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 low AF FB (ST FUEL TRIM) value. Reinstall the original A/F sensor, then go to step 9.
     
NO
     
     
     
The A/F sensor (Sensor 1) was causing the low AF FB (ST FUEL TRIM) value. Replace the original A/F sensor (Sensor 1).?
9.
Replace fuel injectors:

Replace fuel injectors
?1.
Turn the ignition switch to LOCK (0), or 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, and let it idle.

?4.
Operate the vehicle under the same conditions where AF FB (ST FUEL TRIM) was 0.79 or less.
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)
Less than 0.79
      

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 AF FB (ST FUEL TRIM) value. Substitute a known-good PCM, then recheck.?
     
NO
     
     
     
The fuel injectors were causing the low AF FB (ST FUEL TRIM) value. Replace all fuel injectors as a set.?