Possible causes

– Intake air leaks
– Faulty front heated oxygen sensor
– Ignition misfiring
– Faulty fuel injectors
– Exhaust gas leaks
– Incorrect fuel pressure
– Lack of fuel
– Faulty Mass Air Flow (MAF) sensor
– Incorrect Positive Crankcase Ventilation (PCV) hose connection

Tech notes

If the P0171 is combined with the P0174 code, it’s very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replaced the air filter and clean the air flow meter. If the problem persist the front oxygen (O2) sensor may need to be replaced.

There is a factory service bulletin for the following Toyota models:
2005