p0420 post cat replacement

All, looking for any advice you may have with this situation. I have a 2006 Taco with the 2.7L 4 Cylinder and 178k miles. A few months back I had the p0420 code pop with a pretty obvious rattling from the rear cat. I took the car to an exhaust shop and they cut and welded two new universal cats in. Car drove fine for about 1000 miles before the p0420 code came back.

Took it back into the shop and they replaced the rear O2 sensor. This time it only lasted about 250 miles before it popped again. However, in that time I had left the area for a few months and the exhaust shop that did the cat replacement went out of business. Hoping it was just a disconnect between the new rear O2 sensor and the old upstream sensor, I went ahead and replace the upstream myself. Uneventful, and the light went away from about another 1000 miles. However, now it's back and popping roughly every 200 miles or so. This entire time p0420 is the only code that has been thrown. Getting good voltage readings off the rear cat, but can't get any readings off the upstream cat with my OBDII. Based on other threads on this forum, i believe that's normal. Looks like the O2 equivalence ratio is staying near 1 and the car is going into closed loop with the A/F STFT staying between -1 and 0.

The only thing I can think of now is that there is a small leak somewhere in the weld for the universal cats, or maybe at the gaskets when they bolted it back on post-weld, but I can't find or hear anything under the car. Other option could be to re-replace the downstream sensor as it looks like they used a bosch and not a denso. Also potentially related, MPG has been down by about 1 to 1.5 since I replaced the upstream sensor. Was hoping it would improve and not get worse. Before I take this to a new exhaust shop and throw money at them trying to figure out what the last shop may have screwed up, figured I'd swing through here first. Thanks in advance for any advice.

 

Could be a leak but could also be that those cats they used weren't good enough and the O2 sensor is reading a bad mixture since they aren't performing as well as the OEM or OE quality cats would.

Definitely put a Denso sensor in first, Toyotas don't usually respond well to other sensor brands seeing as how the ECU is programmed from factory to work around the exact specs of Denso sensors.

 

The cheap cats are cheap because they dont have as much palladium, rhodium, etc in them. The cats are failing the monitor because they dont have enough oxygen storage capacity to mute/muffle the downstream 02 reading.

You went cheap on the cats and now you're stuck, they need to be replaced with good quality ones comparable to OEM

 

I've had good luck with the Magnaflow cats I used as replacements when my 04 needed them a couple years ago.

Knock on wood, have another smog test coming up soon so we'll find out if they still are lol.

 

Ive also found magnaflow to be pretty reliable as well. I use them more on domestic trucks (chevy, ford, etc) but they come with a good warranty through O'Reilly's. No reason they shouldnt work on Toyotas. Just make sure you're getting something that is labeled "oem grade" or "EPA compliant"

 

Yeah they were the expensive ones that were CARB compliant and able to be used here in CA, think I paid $600 for 2 installed at a local exhaust shop. Passed my 1st smog check a couple months afterward with flying colors and have my next one later this year.

 

As tohers have said;
Toyota are finicky about O2 sensors. Use only Denso.

Cheap cats won't work. Factory or universal Magnaflow "designed" for your truck.

P0420 sucks because it covers so much shit & is relatively vague.

 

You got jipped. Magnaflows would be <$200 for a set & install should be ~$125.

 

Correct, they should be a lot cheaper than they are. But no more cheap cats, not in California for OBD-II catalysts anyway. Not for a couple years now.

 

You must have missed the part about them being CARB cert cats...a lot more material goes into those cats to meet the lovely CA requirements

The non-CARB ones are half that price or less, but you can't even get an exhaust shop to install those for you here. They have to be of the smog legal variety.

 

My apologies. I don't do the CARB bullshit.

 

Thanks for the quick responses. I let myself get talked into the universal cats by the shop against my better judgement. I'm not opposed to cutting my losses and just getting an OEM direct fit replacement. Before I do I'll throw a denso on the downstream and see if it helps at all. Anyone have any recommendations on brands for direct fit cats?

 

OEM cats are by far the best you can get but they are very expensive compared to what's available aftermarket. The trade off is that the aftermarket cats will not last as long as OEM.

When my 04 needed new cats I was originally quoted almost $3,000 to have both (OEM) installed by Toyota. So I went with the Magnaflows instead for $600 at another shop.

I'm happy with the Magnaflows but I do not expect them to last half as long as the OEM cats would have, but 2 years in they're still going strong so I'm happy. When I was shopping around, everyone I talked to said that Magnaflow was the way to go if you aren't able to swing the cost of OEM replacements.

And on that note, I would get cats that are made specifically for your truck...not universal ones. You won't take a gamble on throwing another P0420 code that way.