A long story.car 2.5I 82k miles
Got the P0420 3 weeks ago.Guess it was time for an O2 sensor or something. Checked out sources and decided on www.oxygensors.com
Needed another week because of outstanding commitments for funds.I drive 40 miles roundtrip to work.
Car started to lose power and would not rev past 3-4k. Got it to dealer and was told misfire on all 4 cylinders. Had just replaced plugs and wires within the last 3 weeks. So was told the coil pack must be bad. Got coil pack and installed. Same issues.NGK's were not in stock and didn't want the E3's again. Not a bad plug, but you have to change by 30-35K. I didn't do mine til 37K and one of the electrode triangular parts had disintegrated and I was praying that it had burned up rather than running a piece of metal inside the cylinder. I fished with magnetic probe and found nothing. Onwards.
Got NGK's and replaced on GP. Still the same. I remember having a similar issue years ago with a chevy malibu wagon. In that case the car had run rich and burned inside the cat and fused the silica pellets. The cat would get red hot and you had to watch what you parked over! Got it to a mechanic who gutted the cat, reinstalled and all was well.
In these days of O2 sensors, car PCM's etc all is not as simple, but I checked anyway.
Removed exhaust after 1st cat, turned on car and hit the pedal. Whammo, red line as often as I wanted, although with a lot of noise!
End result, the 2nd cat is partially clogged. Am trying to run rods down it now to bust up material. Because of the bends, not sure how successful I'll be. If unable to gut it, I'll take it to a muffler shop and have it cut out and replaced with straight pipe.
Moral of the story. If you get a P0420. Get it remedied before you have other issues.
Will update after I get this resolved, one way or the other.