My CEL came on at the end of last week and was giving a P0420 (bank 1) code. No driveability issuesI was getting this code back in May, but changed out the upstream O2 sensor and it went away. I took it to a local exhaust/muffler shop and had them track the output of the sensors and they were convinced that it is the cat. since the two sensors were simply tracking one another and giving very close readings.

They quoted me $1000 plus labor... I said thanks, but I'll order my own cat and bring it in for you to install. Got an Eastern Catalyst direct fit from Rock Auto for $200... It came in the mail today, but I was going to run a quick errand before calling to schedule the cat install... Cranked the engine... no CEL. Still no CEL this evening.

What should I do? Have it installed anyway? Wait and see? Install it if the CEL comes back?

Is this what Cats do when they are on the way out? or do you think the problem was misdiagnosed?

 

I'd wait a little while and see if the CEL returns again. After all the car is going ok so no inconvenience meantime.
As you have the new cat now, your also ready to go as soon as you can get a booking.
If you have a reader why not see if the code is also still coming up, out of interest.