Car is a 1999 Lexus ES 300 with 187k. Car was throwing code for P0420 for awhile. I took the car in and had the timing belt, plugs,valve cover gaskets changed. When I picked the car up CEL was off but came back on in a few miles. I pulled the code and this time it was P1135 and P1130. I was suprised not to see p0420. I replaced sensor with new one from Sewell. Today after driving it for about 150 miles P0420 popped up again. Where should I begin looking first? How likely is that the cat is bad?

 

The codes P1130 & P1135 is for the upstream O2 sensor. The P0420 could be the converter or the downstream O2 sensor. If you have a good scan tool you can check the operation of the O2 sensors.