Hello,

I have a 99 es 300 with 68000 miles on it. CEL and trac off light are both on and getting a P0420 code bank 1. Does anyone know what this problem most likely is ?/

 

Replace the post cat o2 sensor & reset the ECU, if it comes back replace the cat. (With a high-flow one from ebay).

 

Post cat O2 sensor? sombody at autozone is saying the one before the cat. converter is bad. there are 3 sensors how do I know which one is bad ?

 

the P0420 code means "catalyst system efficiency below threshold." The two upstream sensors do not measure catalyst performance, so diagnosis should start with the downstream sensor after the catalyst, as Phoenix said.