Just replaced the front o2 sensor on my 99 legacy 2.2 and got rid of the p1131 code (front o2 sensor) and now this code pops up (p0420). When I replaced the front o2 sensor I also stumbled across an exhaust leak at the coupler down stream from the cat. I wonder if an exhaust leak is still present, causing the p0420 code. Also while I had the exhaust down I checked both cats by blowing compressed air through one end and observing how much air blew out the other end. I noted that there seemed to be no real restrictions. I don’t know if this is a valid test of the condition of the cat but I figure that at least it means that the cats are not plugged. Can any one out there give me the voice of experience on this matter? I am trying to pass CA smog and dodging the cops so that they don’t see my expired tags is really getting old!

Also I wonder if a contaminated rear o2 sensor (from dirving around with a bad front o2 for a year) would give me a false p04209 code?

I'm assuming you've tried resetting the ECU right?

a leak can get you a PO420.

a leak can get you a PO420.

 

Very much so.

Repair the hole (weld, braze, wrapthecrap, JBWeld the hole), pour some Seafoam in the gas tank, reset the code.... you're good to go.

 

Later,

Peter

What’s up with the sea foam? I have cleared the codes and the check engine light went out for a day or so before the p0420 popped up. I wonder if driving around with a bad front 02 sensor for the past year has created a carbon build up on the rear 02 sensor causing the new code. Any thoughts?

All right after checking a few other threads on the p0420 topic I have decided that I need to eliminate the possibility of a exhaust leak first and then spend money on parts second. So here is the big question. How do you find any and all exhaust leaks? What’s the trick of the pros that works every time? My car is loud, clunky, ticks and rattles like a tractor. This makes listening for a leak imposable. I have tried numerous times. The idea of going to a mechanic scares me. Poor, desperate, and paranoid, please help!

Jack the car up and put it on 4 jack stands. Do this when the engine is "cold" Start the car, get underneath and run your hands around the exhaust piping and feel for exhaust leaks.