P0171 2001 Toyota Corolla -

Question

Hi the thermostat been replaced the front o2 sensor was replaced and mass air flow sensor has been replaced and the check eng light came on po171 again

Answer 1

check for intake leaks is the first step (sometimes it is hard and you may have to take it to a shop that has a smoke mechine)

next check fuel pressure it will have to be 44 - 50 psi at idle, if its not replace the fuel pump.

see other things to check below

Coolant temp sensor also plays a big role