Random P0420 Cat Efficiency Below Threshold

Well last night I had the first check engine light on my 2nd gen 05 Tacoma 4cyl 2TR-FE 5-spd manual. 104k miles into it's life, I pull the code and it's P0420 Catalytic Converter Efficiency Below Threshold. I cleared codes to see if it would come back.

Drove over 100 miles today with a mix of city and expressway. Never had it come back. Nothing was out of the ordinary last night, anyone have any thought on why this would just pop out of nowhere and not pop again? Should I be concerned about any particular piece of equipment and keep in mind it may be failing soon? Anyone else experience something similar?

Thanks for any assistance you can provide. I'm hoping it was just some fluke. Nothing involved with such a code is a cheap fix, unless it's an exhaust patch. I doubt it's a leak since the code wasn't thrown again though.

 

FWIW -

I got the dreaded P0420 code about 2 months ago. Like you, no difference in performance but just the code. I would clear the code but almost every 40 miles, the code came back. I did the reading here, so I kept looking for other tell tale symptoms. Nothing.

I took the vehicle to Toyota for a read of the code and they had the tech run a diagnostic. Diagnosis: two cats and the O2 sensor behind the last cat. They quoted me a fix: $3000 plus tax. I let them know that the first cat was replaced 12 months earlier, and although not an OEM replacement (mine was stolen so I got the replacement less my deductible), it seemed to be running fine. They suggested replacing the rear cat and the O2 sensor for around $1800. I said no thanks a second time. I suggested replacing the O2 sensor; they said it wouldn't help.

So...I went local to have the code read and get a suggested fix. They confirmed the bad cat but said bring it back for a more in depth diagnostic. At that point, I figured I'd get the O2 sensor on my own, do that, and see what to do next (if anything). I bought the sensor and went to do the repair last Saturday. To my amazement, I found that both nuts that hold the flange and the sensor to the inlet on the exhaust pipe had disappeared. I was able to push the flange and pop it off the stems with very little pressure. The O2 sensor came right out.

So... I went to Toyota, bought a replacement gasket, chased the threads on the stems, put the old O2 sensor back in, and turned on two new nuts. Total cost less than $8. Almost 100 miles and so far, no codes.

Not all fixes will end this happily (at least so far) but I figured posting this here for all you out there who might be facing a similar issue. I don't know if the dealership understood the problem or just reliance on the 'computer diagnostic' but I do wonder had I taken the offer (he was going to take $300 off the price if I left the truck) if they would have made the same discovery and stopped the full replacement. Me thinks: fat chance of that!!!

 

Are you talking about the connection point between the header/manifold and the cat pipe or are you talking about the sensor itself? Those are the only locations I can think of that would have bolts near here.

 

The sensor is the last one after the second cat (gen 1). There is a small flange welded to the exhaust pipe with two bolts that hold the gasket, sensor and two nuts. Essentially, the nuts disintegrated, the gasket failed, and I had a leak (not detectable by sound). Running up to Glastonbury this a.m. so the test continues. So far, so good.

 

Oh, this is a gen 2. The second sensor which causes this code is between the 2 cats. The 2 cats are all part of 1 pipe.

 

Have any exhaust piping crushed?

Mine popped on after I flattened some piping on the rubicon. But my cats might just be toast.

 

Hmmm...might be the restricted flow of the crushed pipe mimics a clogged cat and throws the code.

At 250 miles and the CEL hasn't come on. I might be in the clear!!

 

I was underneath it the other day and nothing looks crushed, just rusted. I'm thinking it's actually the cat going bad since I have a programmer installed that's had it running richer than stock for a good 60k miles now. Perhaps it gets a little too rich of an exhaust stream that the cat can't handle. In some time I'll get back under it and play around with the sensor and looks for leaks.

Keep in mind, the second sensor is between the 2 cats, only the first one would cause the code to pop.

 

I had a P0420 code that randomly showed up in July on my 1st gen, 177k miles. Checked everything meticulously along the entire exhaust line, hit the cats with a rubber mallet to listen for loose ceramics, and measured the temperature going in and out of the cats. Nothing looked out of the ordinary. Ended up buying the Denso O2 sensor behind the second cat since it hadn't been replaced before...sure enough, no more code since then.

 

Are you trying to indicate that only the sensor BEFORE both cats would cause a P0420 to set? Just want to clarify what you are saying.

 

No, what I was saying is the first cat and/or second sensor are the cause of this code. Nothing monitors the second cat. The first sensor is prior to the cats. An unusual difference in readings is what causes the code. A bad cat, a leak in the wrong place, a bad sensor, apparently crushed pipe and an unsecured sensor are all possible causes for this code.

 

Oh I know all the things that cause the code. I have been dealing with this code for years on different vehicles. I was just making sure what you were indicating. It sounded like you were saying that the first SENSOR was the only one that could set the code. That is why I was asking for your clarification.

 

No worries mate. I solved the problem with a sim until I decide on the bigger picture.