Trouble code P0420

Just wanted to throw this in there, check your spark plugs! I had gotten the code a while back after replacing my o2 sensors not so long before that. The tips on my plug were all worn away leading to an in-efficient burn of fuel! This was what was giving me constant p0420 codes. Changed the plugs and the CEL actually cleared itself after a day of driving without me having to wipe it with the scanguage. Good luck!

 

there is a sticky on this... the OP did a fantastic explanation on the causes.

 

At 630,000 miles I’ve had this code 3 times,

It’s always been the 02 sensor.

Cat is still running with no issues,

Stay with the DESNO OEM part.

You can buy them DENSO.com to make sure your not getting a fake one at a bargain price

 

Got the 420 code... checked after reading some ideas here and sure enough, exhaust leak...right after the second cat...where it joins the pipe. (I had gotten a new muffler 5 months earlier)
2003 2.7 litre, 131k, downstream O2 sensor replaced at 80k