Originally Posted by Kozman011
Just had this same thing happen last month on my 2006 Acura TSX with the 2.4. In my mind, the best motor ever created by Honda. I had 238K when this code popped up. Cleared code, came back 50 miles later. Hooked up the Torque app and saw that the downstream O2 was switching very erratically. Changed that out. The erratic readings went away. I noticed my short-term trim readings were a bit high, so I changed out the upstream O2 to see if it would effect it. ST trim dropped by a few percentage points. The code has never came back. Car has never burned a drop of oil and is much, much more responsive since the O2 change out. Start simple. The two O2 sensors (Bosch and Walker) were about 150 total from Amazon. Easy change out. If it wasn't the sensors, I would have just cut the cat out and replaced with pipe.
I tend to agree with Kozman011 ... all good points. If you've never changed either up or downstream 02 .. .you're past due. I'd suggest using Denso sensors only on your Honda though... readily available on Amazon. And on the converter ... I just replaced the one on a 2004 Accord Sedan 2.4L with Walker #16299. Paid $140 or so on Amazon ... is a very good quality converter. Some of the cheaper ones only work for a month or so, and your P0420 code is back. Other than dealing with rusty bolts, was a direct fit. And that would definately be the time for a new downstream sensor, since it's in the middle of the converter.
Other than the converter, you'll need front and rear gaskets, and a set of bolts from the hardware store .... 3 on front and 3 for rear. Good luck.