P0420 and P0430 possible fixes. They worked on my 2005 Toyota Solara 2nd Gen 3.3L MFI DOHC 6cyl.
First get yourself an OBD2 Scanner Code Reader. I paid $ 20. Next, clear codes, run your car to a little less than a quarter tank of gas, put cat cleaner and follow directions on the bottle. Refuel to about half a tank of gas, drive till less than a quarter of tank and repeat the cat cleaner process. Next, let's get your car monitors ready. You must have all of your monitors ready to pass smog. Any time you disconnect the battery or clear codes your car monitors become inactive till a proper drive cycle is performed which is a pain in the but and sometimes doesn't even work. There is an easier way to get your monitors ready: Refuel to less than three quarters of the tank, let your car sit overnight, start the car in the morning, make sure you are in a well ventilated area and all of your accessories are off. Let it idle for fifteen minutes then step on the gas in park till you reach 3k rpms and hold it there for 3 minutes, you may after repeat this two to three times to work, it tuck three times to get my Toyota monitors ready. Make sure all of the fluids are at a normal level and space the procedure over three different days, you don't want to stress or overheat your engine by doing this consecutively. Hope this will help people with the dreaded P0420 and P0439 codes. It worked for me and has been over a thousand miles since I have performed this procedure on my Toyota Solara and the CEL has not come back on.