Possible Causes

WARNING: This page is about a different car, the 2003 Toyota 4Runner. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: When DTCs P0171 or P0174 is recorded, the actual air-fuel ratio is on the LEAN side. A low heated oxygen sensor (S 1) voltage of 0.4 volt or less could be caused by a lean air fuel mixture. Check for conditions that would cause the engine to run lean.

DTCs P0171 and P0174 is set when air-fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on LEAN side (2 trip detection logic). Possible causes are:

Air induction system malfunction.
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 A/F sensor heater.
Defective A/F sensor heater relay.
Defective Positive Crankcase Ventilation (PCV) valve hose or connection.
Vehicle ran out of fuel.

NOTE: When DTCs P0172 or P0175 is recorded, the actual air-fuel ratio is on the RICH side. A high heated oxygen sensor (S 1) voltage of 0.55 volt or more could be caused by a rich air fuel mixture. Check for conditions that would cause the engine to run rich.

DTCs P0172 and P0175 is set when air-fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on RICH side (2 trip detection logic). 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.
Defective A/F sensor heater relay.
Defective Positive Crankcase Ventilation (PCV) valve hose or connection.
Defective ECM.
Vehicle ran out of fuel.

NOTE: If the total of the short-term fuel trim value and long-term fuel trim value is within plus or minus 35 percent at 167°F (75°C), the system is functioning normally.