YAY! Another P0420 Code.

So I'm working on my parent's 1998 Honda Accord with the 3.0 V6.

Code P0420 code came up. Now, I have seen my share of P0420 codes, and in my opinion, it is HANDS DOWN, the most misdiagnosed code ever (this is in general, not just on Hondas). So I began testing. Passed the rattle test, so i began more in depth testing.

I hook my Vantage up to it, and driving down the road, I can watch the O2 sensors. At a stead speed like 45 or even 60, the upstream has good activity, just running up and down like it should. The downstream sensor, it seems to sit at about 600mv, which is to be expected as it would have likely consumped it OSC and will tend to run just barely rich at a cruise, and this has usualy been a pretty good indication of converter health for me.

However, sometimes, at about 60 mph, the upstream will go "lazy" maybe 3 or 4 switches in a 5 second sweep, and the downstream will mimmic it alomost like a mirror image. I also see alot of that happen at low speeds around town. Almost like it cannot get hot enough to keep the cat lit off and stops burning like it should.

I'll be honest, I'm ASE certified in A-1 through A-8, and I am an instructor for Auto Service Technology, and switch ratio and OSC testing a converter and calling it bad with an O.E. price tag of 1,066.81 is STILL the diag I hate the most. These tests almost ALWAYS seem "fuzzy".

I just thought I would get the input of some of you guys who know and love these cars, probably know them alot better than I do.

Should I try tossing an upstream O2 at it before I tell them the need to drop over a grand on a converter?

I appreciate your input and help guys, you are very knowledgable and have been very helpful in past.

 

The ECU was pretty accurate on my Accord. Replaced cat, reset the code, and the CEL went off.

Oh, and I'm also talking with www.converterguys.com recyclers to get $220 back for the old cat

 

Does the engine run fine? I mean im no ASE certified guy, but normally a clogged converter willl result in the engine running like crap(Too much back pressure). Smell bad? You looked at teh cat after a good long frive, especially at night? Is it glowing, from being so hot? What about the manifold, is it glowing(After a long drive)?

Rockauto has converters for about $150. O2 sensors are like $21, plus shipping of course. Seems like youve done alot of troubleshooting, interesting how the back O2 sensor follows the front one. I hate throwing parts at a problem, but from what youve already done, I personally would throw in an O2 sensor first, then go from there.