So i got a P0420 cat below threshhold on my 02 with 116K. Ive done searching and i see a few different suggestions. alot of people just say replace the cat which i believe is not the way to go about it lol. id rather properly diagnose the problem. ( this shits like a game to me lol)

so i see some threads on people saying the cat breaks up and shears the head off the 02 sensor.

so my first step should be remove the 02 sensor and check to see if its intact? if its good, replace cat ( with test pipe ) and if its sheared off, replace 02 sensor and cat?

any other steps i should consider? maybe test the front 02 sensor for proper voltage? although i dont suspect it as theres no code for oxygen sensor circuit.

 

There you go, I believe the normal operating range in .2 and .8 volts. The engine must be warmed up. But I always say when something breaks its time to upgrade. I would get a TP, CEL eliminater, and a new sensor.

 

Primary sensors rarely go bad.
Secondary sensors (in the cat) more so.

I'd probably insert one step in the middle:

Remove and shake catalytic converter. If it doesn't chunk or rattle, it's probably not the problem.

 

p0420 is the cat...

 

just because the cat doesnt rattle when you shake it doesnt mean its not bad..

i have done plenty of cats that dont break apart and cannot properly react to the exhaust gases causing the rear o2 sensor to act like the front and crosscount, hence causing the p0420..

more than likely that cat is bad.. i rarely see a rear o2 sensor faulty and cause a p0420