I'm new to this site so I hope I am posting in the correct spot. I wanted to share my experience this week with a PO420 (catalytic converter bank 1, something like that) code that I have been trying to figure out for some time. I want to spare you a lengthy explanation so I will just list the steps I took. The PO420 code is a tricky (removed) and I am by no means a mechanic, but I have youtube and I love taking stuff apart.
At first glance it seems that PO420 would be something to do with the CAT itself, but after researching I found that a faulty CAT is rare, even in older vehicles (My vibe is at 126k). I also read that this code can be triggered by sensors in the engine before the CAT even receives the fuel mixture/exhaust. It had been some time since I had serviced the vehicle so I went ahead and changed the spark plugs (much overdo) and in doing so found that coil #1 was not functioning, so I replaced coil #1 as well. So now my vibe is really humming, she's not my main ride so I haven't paid her enough attention lately. So i reset the light via disconnecting the battery. Now, on my way to school and about 30 miles into my commute the friggin' light clicks back on. So now I'm thinking its definitely the CAT, because what else, right? I had recently replaced the forward 02 sensor near the manifold so I knew that wasn't the problem. I figure either the downstream 02 sensor or the CAT. Looking at the engine, my air filter was filthy, replaced it. I'm still not confident, so I call my uncle (mechanic) and ask him. He tells me to replace the Mass Airflow Sensor because a dirty filter can damage the original over time. Presently, I am at about 30 miles in and no light , since I reset it after replacing the MAF-S. I am keeping my fingers crossed because I need this one to pass inspection. I hope that my experience can help someone else dealing with this code. I will update this post if anything changes. Please let me know if you've ever encountered this problem.