CE light & code for 0420, CAT or O2 sensors?

0420 Code - bank one below threshold - 'grey' code.
O2 Sensors were replaced about 70K miles ago.
I have 236K on the original exhaust.
Could the cat(s) be clogged? O2 sensors are just over $100 on Amazon, may try that first.
Any suggestions on what I should anticipate a replacement cat to cost?
If so, I see cats on Amazon for $50-$300 (for CA emission that I don't have to worry about).
Thanks!

 

Bowyer,

I would check your O2 sensors before replacing them....easy test, just google it, may save ya some money. Testing the cat is a little more involved. You need a back pressure test set, ideally, or you could use a vacuum gauge at the intake. The cats from the dealer here in FL are $1033 each, you can buy after market though for much cheaper....although I couldn't speak to the quality compared to OE, but a grand is a big check to write for a cat. Once again I would not replace parts until you isolate the problem. Go to a shop and eat the 100 bucks to have the cats tested before buying new. Start by checking the O2 first

 

I got the 0420 code on my truck at about 252k miles. Turned out to be the vacuum hose that goes from the air box to the other side of the engine to a VSV. It was torn at the airbox but I couldn't see it. Replace hose with a new one and code went away. Worth a look