Maybe I'm a fool, sorry if that's the case.

But I'll cut to the chase. I'm throwing a P0420 code that shows up every now and then on my 01 Limited. I live in state that doesnt do smog testing . I'm wondering if this is actually a problem with my cat reaching the end of it's like (221,xxx mi) or is this just a weird 02 problem. I don't have countless dollars to throw at the problem and replace parts until I found the right one such as I would like to......so can I dodge this problem easily buy zeroing in on ONE particular issue...or....what?

Help. lol

Any exhaust leaks or 02 gaskets leaking? Especially near the cat converter.

Besides leaks can be a faulty O2 sensor or a bad cat.


They also make a cat clean additive for your gas that can clean a clogged cat.

However, if the cat is screened burned out. Then the cat will need to be replaced.



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For me it was antifreeze.

Even a small amount where you only top off your overflow tank once every few months will cause it.

Cats don’t really ware out under normal use. Overheating from missfire’s will melt them.

I have the OEM cat that at 230,000 ish miles started giving me the dreaded 420 code. I tried everything including a rear O2 simulator.

When it became obvious I had an issue with antifreeze consumption I ended up rebuilding my spare engine. I decided to listen to a few sites about what causes the 420 code and put the same cat back on. The O2 readiness monitor set on my first drive to work. It’s been fine ever since.


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It can be caused by bad O2 sensors, cats, MAF, or other problems, but is mostly caused by exhaust leaks around the cat or the O2 sensors.

I have the same code.
Mine is being caused by my bad gasket in the flange connecting the cat convertor to the exhaust pipe.
Mine is making a ticking sound and I can also feel exhaust air coming out of it so I would say check first for leaks in your manifold all the way down to your tail pipe.
But like others have said it can be caused by a few other things but keep your fingers crossed it isn't the cat convertor!
Best of luck

Mine was the cat failed and once replaced at 275k the prb was fixed.

Mine was thrown by a bad O2 sensor receiving like 4v or something. If you can, check with your OBD how much power they're getting, iirc it should be 12v. Can't go wrong with working small and starting from there.
Forewarning: they're known to be rusted on beyond belief and often halfway brazed to the exhaust. I'd recommend spraying a bunch of PB on them and letting them soak for a while (helps if they're warm too when you loosen them) but don't be shocked if you gotta buy new nuts. Maybe get some beforehand if you have no other ride.

My 01 and others on here have mentioned if we drive 1-10 mph, usually while off roading, for just a mile or two and Bam, there is it pending 0420, like clockwork, everytime. My state does smog test and it passes every year.

I can drive for tens of thousands of miles but one slow speed mile or two and there it is....

I haven't been able to pin down any places with exhaust leaks, at least as far as I can see. There could possibly be a crack or something up close to the manifolds, I never stopped to think that the error code could have actually been caused by a LEAK.

This i'm curious about. You mean your engine was ingesting coolant somewhere through the system, or just a being below a certain amount in the cooling system is causing this code to pop up.

You know, my girl's youngest son is like Rain Man and can pick up on tiny things that I'm too deaf or otherwise distracted to notice, and pointed out a very minor ticking sound........I'm starting an investigation in that area.


Thanks a LOT for all the quick responses and help. I'll go attack this task list and report back

You know, I did start to notice this popped up directly after wheeling, but it had been doing it on and off since I bought the truck lol

The 96 engine I put in my 97 was ingesting antifreeze from a bad cylinder. What I thought was a bad head gasket ended up being far worse and required me to rebuild the engine I pulled out of the 97.


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weren't driving out to colorado were you