Possible Causes

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

These DTCs use two-trip detection logic.

DTC P0171 is set when air fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on LEAN side. Possible causes are:

Air induction system malfunction.
Defective Positive Crankcase Ventilation (PCV) valve hose or connection.
Fuel injector restricted.
Defective Mass Airflow (MAF) meter.
Defective Engine Coolant Temperature (ECT) sensor.
Improper fuel pressure.
Exhaust system leak.
Air/Fuel (A/F) sensor circuit is open or shorted.
Defective A/F sensor.
Defective EFI main relay.
Vehicle ran out of fuel.

DTC P0172 is set when air fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on RICH side. Possible causes are:

Fuel injector restricted or leaking.
Defective Mass Airflow (MAF) meter.
Defective Engine Coolant Temperature (ECT) sensor.
Ignition system malfunction.
Improper fuel pressure.
Exhaust system leak.
Air/Fuel (A/F) sensor circuit is open or shorted.
Defective A/F sensor.
Defective A/F sensor heater and relay circuit.
Defective EFI main relay.
Vehicle ran out of fuel.