I have a '97 Lexus ES300 with 112K miles. I had got a CEL with P0135 code. I replaced the bank 1 sensor 1 O2 sensor with a OEM sensor. Two days later, I get another CEL and now the code is P0420 - catalyst efficiency below threshold.
I had the P0420 code about 6 months ago and I had replaced the cat. Since then P0420 code was gone and everything was fine.
Any ideas why I am getting P0420 now, after P0135 and after I replaced the O2 sensor? What should I be replacing now?
Thanks, Mark