Originally Posted By: greenjp
The wife's 2006 Accord with about 55k miles on it just turned up code P0420 today, catalyst efficiency below threshold (bank 1). A little research indicates this probably means either the post-cat oxygen sensor is bad, or the sensor is fine and the cat itself is actually going bad. Any thoughts on those, how to check, etc?
Also, am I correct in believing this ought to be covered by the mandated extra warranty on the emissions control system?
You are correct. The federal EPA warranty is 8 years or 80,000 miles, whichever comes first. That's a ONE-TIME ONLY warranty, so subsequent cats are subject only to the maker's own warranty. This is explained in detail on the EPA's website.
It's not the downstream oxygen sensor, so forget about that idea. Unless there's an exceptional issue, the cat is simply bad.
You MUST insist the dealer check for TSBs for your vehicle and P0420 DTCs. They are unlikely to check on their own, and you'd be surprised what can lead to a P0420 that originally had nothing to do with the cat. I have a page on my site where I compiled a list of TSBs outlining problems that led to cat damage, and needed to be corrected before the cat was replaced. See at the bottom of this page:
http://www.tegger.com/hondafaq/p0420/index.html
If you've been careless enough to allow the car to run out of gas once or twice, or to have driven for extended distances with a very low fuel-level, don't tell the dealer. That is considered abuse, since it causes substantial damage to the cat, and it may complicate the warranty claim. There are warnings about such behavior in your Owner's Manual.
Finally, DO NOT HAVE AN AFTERMARKET CAT INSTALLED, and DO NOT allow an independent shop to work on the car for this issue. Those would be the WORST things you could do right now!