Question

I failed emissions with a po171 and po120 code--what are these? 1999 Toyota 4Runner

Answer 1

P0171 TOYOTA - Air/Fuel Mixture System Lean Bank 1

Possible causes: 

check to see if air filter is clean first

- 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

https://www.autocodes.com/p0171_toyota.html

P0120 TOYOTA - Throttle Pedal Position Sensor/Switch 'A' Circuit Malfunction

Possible causes:

Very common problem with Toyota models. Before replacing any parts, try cleaning the throttle body and reset the light. If the codes come back, then it will necessary to replaced the throttle position sensor (TPS). Most auto parts sell the TPS for around $60.

- Throttle position sensor circuit is open or shorteded 

- Throttle Position Sensor poor connection 
- Throttle position sensor may be faulty

https://www.autocodes.com/p0120_toyota.html