Fuel Supply System

(_P0171_) The scan tool indicates Diagnostic Trouble Code (DTC) PO171: 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 in two consecutive trips, the MIL will come on and DTC PO171 or PO172 will be stored.

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

P0135: Primary HO2S Heater
PO136: Secondary HO2S
P0141: Secondary HO2S Heater
PO4O1: EGR Flow Insufficient
P0441: 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 insufficient fuel pressure, amount of flow
- Fuel Feed Line clogged, leaking
- Fuel Pressure Regulator stuck open
- Fuel Filter clogged
- Fuel Injector clogged, air inclusion
- Fuel doesn't meet Owner's Manual spec.
- Primary HO2S deteriorated
- MAF Sensor range/performance problem
- Air Cleaner (ACL) element damaged or dirty
- EGR System not enough flow
- Valve Clearance
- Intake air hose loose, leakage
- Vacuum hoses leakage
- Exhaust System leakage

(DTC P0172
System too rich)
- Fuel Pressure Regulator clogged, stuck closed
- Fuel Return Pipe clogged
- Fuel Injector leaking
- Fuel doesn't meet Owner's Manual spec.
- Primary HO2S deteriorated
- MAF Sensor range/performance problem
- Air Cleaner (ACL) element damaged or dirty
- EGR System malfunction (too much flow)
- EVAP Purge Control Solenoid Valve leaking stuck opened
- Valve Clearance