Fuel Control System

P0171:
The scan tool indicates Diagnostic Trouble Code (DTC) P0171: The fuel system is too lean.

P0172:
The scan tool indicates Diagnostic Trouble Code (DTC) P0172: 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 during two consecutive trips, the MIL will come on and DTC P0171 and/or P0172 will be stored.

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

P0106-8: MAP Sensor
P0135: Primary HO2S Heater (Sensor 1)
P1037-8: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S Heater (Sensor 2)
P0401: EGR Flow Insufficient
P0441: EVAP System Incorrect Purge Flow
P1259: VTEC System (H22A1 engine)
P1491: EGR Valve Lift Insufficient
P1498: EGR Valve Lift Sensor High Voltage

Possible Cause
DTC P0171: System too lean
- Fuel Pump insufficient 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 deteriorated
- MAP Sensor range/performance
- EGR System insufficient flow
- Valve clearance
- Exhaust leak

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 deteriorated
- MAP Sensor range/performance
- EGR System malfunction (too much flow)
- EVAP Purge Control Solenoid Valve leaking, stuck opened
- Valve clearance