My 2003 Corolla with 270K has had the P0240 cat efficiency code in the past, and I got rid of that one with the spark plug non-fouler on the downstream trick. It then threw the dreaded P0171 code. It runs fine other than a rough idle, so I've ignored it but now need to pass county emissions testing to renew the license. Tried this so far;
Replaced intake gasket with new type orange gasket
Replaced throttle body gasket
Replaced air filter
Replaced MAF sensor with Amazon $20 cheapy
Had replaced both O2 sensors a couple years ago with Denso units.
Replaced several vac hoses that were hard and cracked.
Replaced spark plugs.
Still throws the code. Fuel trim readings are high, from what I've read it most likely is a vacuum leak.
Replaced intake gasket with new type orange gasket
Replaced throttle body gasket
Replaced air filter
Replaced MAF sensor with Amazon $20 cheapy
Had replaced both O2 sensors a couple years ago with Denso units.
Replaced several vac hoses that were hard and cracked.
Replaced spark plugs.
Still throws the code. Fuel trim readings are high, from what I've read it most likely is a vacuum leak.