P0420 code

I also own a 2006 Prius (206,000 miles) with a CEL. How would I go about eliminating the cause of the CEL systematically? For example, if I put in CataClean or SeaFoam, or both, I can get the light to turn off for a short while and then it comes back on by the time I run through a tank of gas. Does this point to one cause or eliminate any of the possibilities? I don't want to dump $1000+ into the car for a new CAT or lose $1000+ on resale because I am getting ready to sell the Prius after having successfully maintained it and otherwise taken great care of it.

I do suspect Valvoline may have overfilled my oil during an oil change once (4 full quarts of max life synthetic poured from bottles instead of precisely 3.8 quarts from the pump -- could .2 quarts make that much of a difference?). It started intermittently first but since became permanent.

 

Do you have the P0420 code? How many miles on your Prius? How much oil does it consume? Do you need to add oil between oil changes? What interval are you changing the oil?

If you have P0420 and you have good O2-sensors (other one is actually air fuel ratio sensor) you can try to clean the CAT with something but it won’t probably be long term fix. Then your only option to get rid of the code, other than replacing the CAT, is to fool the ECU into thinking that CAT is working fine, which might be illegal.