DTC P0172: Fuel Trim, Too Rich: Condition

WARNING: This page is about a different car, the 2002 Mazda Protege5 and 2002 Mazda Protege. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when short term or long term fuel trim values exceed pre determined value while Drive Mode 1 - PCM Adaptive Memory is running. See DRIVE MODES under SELF-DIAGNOSTIC SYSTEM. This is a continuous monitor. MIL is illuminated when PCM detects malfunction during 2 consecutive drive cycles. Possible causes are:

Front HO2S deterioration or malfunction.
Fuel pressure regulator malfunction.
Fuel pump malfunction.
Fuel return line plugged or restricted.
Misfire.
PCV valve malfunction.
Pressure Regulator Control (PRC) solenoid valve malfunction.
Purge solenoid hoses improperly connected.
Purge solenoid valve malfunction.