Fuel Supply System

[PO171] The scan tool indicates Diagnostic Trouble Code (DTC) PO171: The fuel system is too lean.

[PO172] The scan tool indicates Diagnostic Trouble Code (DTC) PO172: The fuel system is too rich.

Description
By monitoring the Long Term Fuel Trim, long term malfunctions in the fuel system will be detected.
If a malfunction has been detected in two consecutive trips, the MIL will come on and DTC P0171 or P0172 will be stored.

NOTE: If some of the DTCs listed below are stored at the same time as DTC PO171 or P0172, troubleshoot those DTCs first, then troubleshoot DTC P0171 or P0172.



PO1O6-8: MAP Sensor
P0135: Primary HO2S Heater (Sensor 1)
P0136: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S Heater (Sensor 2)
PO4O1: EGR Flow Insufficient
PO441: EVAP System Incorrect Purge Flow
P1491: EGR Valve Lift Insufficient
P1498: EGR Valve Lift Sensor High Voltage

Possible Cause:

[DTC P0171: System too lean]
^ Fuel Pump: not enough flow/pressure
^ Fuel Feed Line: clogged, leaking
^ Fuel Pressure Regulator: stuck open
^ Fuel Filter: clogged
^ Fuel Injector: clogged, air inclusion
^ Gasoline: doesn't meet Owner's Manual spec.
^ Primary HO2S (Sensor 1): deteriorated
^ MAP Sensor: wrong characteristics
^ EGR System: not enough flow
^ Valve Clearance: too tight
^ Exhaust Lines: leaking


[DTC P0172: System too rich]
^ Fuel Pressure Regulator: clogged, stuck closed
^ Fuel Return Pipe: clogged
^ Fuel Injector: leaking
^ Gasoline: doesn't meet Owner's Manual spec.
^ Primary HO2S (Sensor 1): deteriorated
^ MAP Sensor: wrong characteristics
^ EGR System: malfunction (too much flow)
^ EVAP Purge Control Solenoid Valve: leaking stuck opened
^ Valve Clearance: too loose