Possible Causes

WARNING: This page is about a different car, the 2003 Toyota Camry. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0171

Defective air induction system.
Injector blockage.
Defective mass air flow meter.
Defective engine coolant temperature sensor.
Defective fuel pressure.
Gas leakage in exhaust system.
Open or short in A/F sensor circuit.
Defective A/F sensor.
Defective A/F sensor heater.
Defective EFI relay.
Defective (Positive Crankcase Ventilation) PCV hose connection.
Defective PCV hose.

DTC P0172

Injector leakage and/or blockage.
Defective mass air flow meter.
Defective engine coolant temperature sensor.
Defective ignition system.
Defective fuel pressure.
Gas leakage in exhaust system.
Open or short in A/F sensor circuit.
Defective A/F sensor.
Defective EFI relay.
Defective A/F sensor heater.