Help me diagnose my P0420 code (chugging noise?)

Hi all,

Truck: 2001 Tacoma, 3.4L V6 auto, 318,XXX miles
Code: P0420
Symptoms: decreased fuel efficiency, maybe lower power (?), chugging sound when in gear and warmed up

Purchased this truck a few months ago, and unfortunately, the CEL turned on before I got a chance to do emissions/register. My generic Amazon code reader is showing: P0420 Catalyst system efficiency below threshold bank 1, which sounds like it means I have a bad cat. From my reading, it seems like an exhaust leak could also cause this code, but the exhaust seems to be in good shape as far as I can tell (no rust). I've reset the code twice. The first time, it stayed off for about a tank (200 mi), and the second time it returned after ~350 mi. I have also noticed a few other changes that may or may not be related:

- Decreased fuel efficiency (~14MPG)
- Possible loss of power, but this may be due to the 265/75/16 Load E tires I installed?
- Blowing noise/low power right after startup. Quickly resolves itself.
- Chugging noise when in gear, only appears after the truck is warmed up (8 or so minutes?)

As you know, the 01 has two cats, and I am HOPEFUL that I don't have to replace both of them. I read online that you can use an infrared thermometer to get a reading before and after the cats and identify the bad one that way. Supposedly, a bad cat will be hotter before the cat compared to after it. Unfortunately (or fortunately?), both cats were hotter at the back end, suggesting that they are at least semi-functional? Additionally, the cats are free of rust and don't make any noise when I knock on them with my fist. So, I seem to be back at square 1, without a way to diagnose which (if either) of my cats has gone bad.

Would greatly appreciate some advice as to what my next steps should be. Thanks!

 

PO420 is always a bad cat (in my experience). The chugging could be due to the cat being clogged up or falling apart inside. I've worked on one that was so clogged it was causing misfires, because the exhaust wasn't flowing.

 

This seems to be the consensus of the internet as well. Any recommendations for the best way to diagnose which one is bad? Is it surprising that the temperature readings seem normal (>50F hotter at the rear)?

Thanks!

 

New cat converters solved my recurring P0420, same symptoms you describe too.

The other common thing to look for would be an exhaust leak before the O2 sensors, making them think the catalyst isn't efficient anymore.

People like to throw new O2 sensors at this code but then it ends up coming back.

 

Ah well, that's good to hear. I'll be glad to be rid of that chugging. Did you end up replacing both of them? OEM or aftermarket? Thanks

 

Aftermarket, OEM are crazy expensive. OEM is the best you can get though and will outlast any aftermarket brand, but you'll pay for it

I went with Magnaflow, they're widely regarded as the best aftermarket brand you can go with but there are others. If you need to pass smog (like I do in CA) then make sure you get the 50 state legal ones.

In fact, I would probably get those anyway because they are the closest to OEM than the ones that aren't. More of the expensive material in them to filter the exhaust gases which is what makes them cost more.

Some of the dirt cheap aftermarket cat converters might still trip the P0420 code after install if they don't satisfy the O2 sensor

 

My pending 0420 code never came back after showing up a few times when I drove through areas with crappy fuel.

Could for sure be an actual bad cat.

However, if you want to start inexpensive first OP, maybe run a few fuel system cleaners through the truck with good quality fuel as well and monitor it for a month or so. Might get lucky enough to pass emissions. But if the chugging and power loss don't improve I'd dig deeper.

Ay idea how old the plugs are, or how long it's been since the truck had some TLC done to it in general?

 

Do you have any service records for the truck? They are notorious for not playing well with cheap sensors. If the previous owner got a similar code, they may have throw some cheap O2 sensors at it. Getting back to Denso sensors would be a priority if you are unsure.

 

It happened on my 5 lug for a while and I’m close to 300k, pretty sure my cat blew out the rear and it for sure sounds different now but I rarely get the code anymore