Dodge van OBDII P0420 code sanity check

[Pre-Edit: Plenty to ponder and prognosticate on, but the central question is whether anybody familiar with the vehicle knows of some reason that crops up regularly besides the cat for throwing the P0420 bad cat efficiency code...]

The van (2001 dodge B2500 with the 318, 140k miles) is complaining about catalyst efficiency. [EDIT: though it would be about the most pertinent info in the world I forgot to mention that both O2 sensors have been replaced quite recently. This code happened a few hundred miles and a month or two after the O2 sensors were replaced.]

Mostly runs great, but I've had a few random no-starts, but generally after turning all the way off and back on it fires up immediately. I know, that one sucks; I was waiting for it to eventually either throw a CEL and give me a hint or get bad enough to start gambling on things like a crank position sensor.

It sits usually a week or so between runs, and can be just a little rough on startup, but not at all bad. New cap and rotor, wires look great, plugs look great (though I haven't pulled all eight).

Basically, there's just enough worrisome about general condition to give me pause before I pop for a new cataclysmic converter. The secondary bummer is that I'd much rather be working on the garage in any spare time, and would much rather just hand this off and have it taken care of at a shop, but I fear paying for the diagnostic process on this one...

Any thoughts?

I haven't done a compression test, fuel pressure test, or actually looked at all eight plugs. So, I've got some stuff I could do, some stuff I could hand off to a shop to do, or I could gamble on the cat.

Sometimes thats a code for a bad rear 02 sensor also.

Monitor rear 02 sensor cycle times and see if they are in check.

Probably is a clogged up kitty that needs a good dose of magnesium citrate.....

I would just go with one of those high flow cats available and just clamp that sucker in place with those fancy wraparound band clamps...

P0420 is legendary amongst Subaru owners. Here's my opinion based on all the reading I've done: It ain't necessarily the catalytic converter. Or it might be.

Here's how it goes in a Subaru...There are two O2 sensors; one up in the early part of the exhaust, then the cat, then a second one downstream. The downstream one's only task in life is to say YES or NO to the exhaust coming out of the cat...if it's clean enough, the light stays off. If it doesn't, the CE light is triggered. Replacing the rear O2 sensor has almost never solved the problem, since it controls nothing and just reads.

The front O2 sensor talks to the computer and does all the mixture adjustments. It is possible that the front sensor goes bad, tells the computer to do wrong things richening the mixture and causing the second sensor to say NO, YOU FAIL, triggering the CE light. Replacing the front sensor will sometimes fix the problem.

My own theory is, if that doesn't fix the problem, then the catalytic may well be contaminated (and on a Subaru, with their head gasket issues, this is not unlikely). This contamination may or may not burn out over time.

On the other hand, I've read of multiple people who have replaced the cat(s) at great expense (both a/m and OEM) with the same result: a recurring P0420 code.

Thus, the invention of the "$5 fix", which is a spark plug anti-fouler adapted to fit between the exhaust pipe bung and the second O2 sensor, thus moving the pesky sensor from the exhaust stream while leaving it functional. This works in most cases, though it eventually proved unsuccessful in mine.

Ultimately, my solution was to just reset the gawddamn light every so often and just before annual inspection. My ultimate-ultimate solution was to sell the car and buy something that didn't have a P0420 code every 2 weeks.

Super-facepalm on my part.

One might suspect the fact that I have recently replaced both the upstream and downstream O2 sensors (a couple of months and a few hundred miles ago) would be pertinent information. Sorry about that. I'll edit the original post, but it's sounding like this one's pretty simple.

I'd much rather shell out for a cat than spend a bunch of time chasing something weird, so I'm not going to call that terrible news

if have only ever seen 420 codes that were bad cats except for 3 times. in 10 years only 3 that werent makes me sure its a cat.

Awesome. I like increased certainty. Makes it a lot easier to part with the dollars...

Ranger50, I'm tempted to schwack one of those in, but at this point I may just go with the time savings of dropping it off at the local GRM-forum-recommended exhaust shop and getting it back fixed.

I still need to find time to do the ball joints, tie rods, and shocks, and piling up more to-dos while the garage is half-done is unappealing...

KATYB wrote: if have only ever seen 420 codes that were bad cats except for 3 times. in 10 years only 3 that werent makes me sure its a cat.

I had a 3.3/3.8 Caravan like that once.... Broken flexplate that berkeleyed with the timing and reduced power big time, plus the blinking MIL with a P0420.....

Ranger50 wrote:

KATYB wrote: if have only ever seen 420 codes that were bad cats except for 3 times. in 10 years only 3 that werent makes me sure its a cat.

I had a 3.3/3.8 Caravan like that once.... Broken flexplate that berkeleyed with the timing and reduced power big time, plus the blinking MIL with a P0420.....

never saw that. i know 2 of them were mustang v6's with a small crack before the second 02 in the exhaust that wasnt audible but leaked just enough to cause issues. third was an audi.

I can cause a 420 on my Focus under the right conditions. Need a wet rainy day. Then be cruising thru town at 25 mph in 4th. Punch the accelerator to get thru a hole in traffic without downshifting equals CEL with 420 code.

I think the supercharger and the high flow aftermarket cat probably have a lot to do with it. I just reset and it may be 6 months to a year before it happens again. Always in the same situation.

my P0421 troubles began after running a six-year-old bottle of octane boost through the tank in my '03 mazda6. not sure what i was thinking, since the car is designed to run on regular...

angry corvair not to mention mazda6's assuming v6 from what i remember runs so pig rich its rediculous and cats are a notorious failure on them. full throttle they do go 9.8:1 af ratio.

my P0421 troubles began after running a six-year-old bottle of octane boost through the tank in my '03 mazda6. not sure what i was thinking, since the car is designed to run on regular...

I gotta be honest. That is pretty funny...