P0420 code, but cat and sensors are fairly new

Hey guys. I recently purchased a 2001 v6 with 167k on it. Yesterday the check engine light came on. I took it to advanced auto to get the codes and I got p0420. Which as we all know means low catalytic converter efficiency. After doing some digging here, it appears that I most likely have:

- A Bad Cat... which I'm hoping its not
- Bad A/F or o2 sensor
- Or an exhaust leak

I was looking at the maintenance records from the previous owner and he had the catalytic converter and o2 sensor replaced 30k ago (in 2014 - yes low mileage the past few years). He also had the MAF replaced 25k ago (2018).

I have an appointment for a shop to look at it. Should I buckle up for a new cat? I feel like it should be something else, seeing as it was replaced 30k ago.

My entire exhaust is rusty as crap, and it rattles when I accelerate. Maybe an exhaust leak? If it is the cat, how much should I expect to pay for a new cat and labor? I don't want to go OEM as Ive heard they are insanely expensive. Is it something a person could do themselves who have basic mechanical skills (I've installed lifts and what not, but never touched exhaust).

Also, how should I approach the shop? I feel like if I just bring it in they are going to say "yep new cat" without looking at other options... Its a toyota specific repair shop, but not a dealer.

Any help can be appreciated.

 

If you've installed a lift you can replace a bolt-on exhaust system. Might end up having to drill or cut out fasteners but that's about the worst of it. See what the exhaust shop says, but for reference I replaced everything behind the headers on my truck in September. Two Walker cats and the JBA catback were $850. If you're in a CARB state those cats get a lot more expensive.

https://www.tacomaworld.com/media/albums/2021-09-exhaust-refresh.68559/

Best case scenario the rattling might be from the cat heat shield. If the rattle is internal to the muffler or cats you'll want to replace those. You can test the flanges for leaks with soapy water. If you're lucky maybe you just need some new gaskets.

 

Did you have the code reset to see how quickly, if at all, it come back?

Do you have to pass emissions testing for licensing/registration?

 

Smoke test might show an exhaust leak. I'd ask for that to be done.

 

420 can be triggered by an exhaust leak up steam.

 

hopefully new gaskets. I might attempt it all myself. Might also have to come back here for parts recommendations and everything I need.

Didn’t reset it. Should I disconnect the battery to see how long it takes to come back? i should also note that I got gas at a mountain town gas safari on that I never fill up at. Light came on after about 50 miles on that stuff. Could this be the culprit?
I do have emissions. Not for another 2 years though. But I want to get it done right.

 

Nothing to lose by resetting it.

 

If I reset it and it doesn’t come back on, should I worry or write it off as bad gas?

 

If it doesn't return, no need to worry. You can write it off however you like. If it comes back on, just place your hand near the flanges while it's running. A leak severe enough to trigger P0420 should be noticeable. The gaskets deteriorate and crumble away. Autozone carries replacements for <$10...get some stainless flange bolts and nuts to reassemble.

The only thing suspicious about all of this is that the MAF was replaced 5k after the cats...it's plausible that a faulty MAF (or some other fault that was attributed to the MAF) caused a rich condition, causing the original cats to fail in the first place. Based on the history, the causal factor may not have been corrected until after the cats were replaced. There is some uncertainty here, but brace yourself for needing to replace cats.

 

Do you have an OBDII scanner? They're so inexpensive everyone should have one and keep it in the vehicle. Walmart sells a little Autel that can do live data, freeze frame etc.. for around $40. But reason I'm asking is because you can check the rear O2 performance and verify a bad cat pretty easily. If you suspect an exhaust leak I've seen some people on the youtubes use a leaf blower or a shop vac with the 'exhuast side' (blower side) shoved into the tail pipe and just using a soapy water solution sprayed at suspect areas and look for bubbles - in lieu of a smoke machine which most people aren't going to have.

 

You'd be surprised how quickly cats can go bad given the right conditions. Our work vehicles go through them pretty quickly due to high idle hours.

That being said, my last truck had a cracked manifold which caused me to have to replace my cat. One way you can test to see if you need a new catalytic converter is getting after it. Merge onto the freeway at full throttle. If you get a flashing check engine light and notice it won't go over a certain RPM, your cat is likely plugged up. Alternately you can bang on the cat and if you hear a rattling sound, your cat is pretty much done for.

 

with a towel or a large rag start the truck and let her idle, take the rag and have a buddy block the exhaust pipe. crawl under the truck and listen for the exhaust leaks. All leaks before the cat(s) need to be fixed.

most people will replace the factory cat(s) with aftermarket cats due to cost. IMHO, this is a false economy. You pay the price for factory cats because there is more of the material that makes them work inside, aftermarket cats are much smaller.

If you live in a "smog state", there are a few things you can do. You will have to do your research on the board about building a rear O2 sensor simulator. You are lucky that you have the V6 as the rear 02 sensor cable (single cat) comes up under the passenger seat and you can hide the modification from the smog tech, unless he is a smog nazi and knows what to look for and where to look. The second is that it takes most trucks a while to trip a P0420 code. If you have an OBDII scanner and you have reset the PCM you will have to monitor your smog monitors and catch it at the right time, all monitors cleared and no codes, take her and get her smogged immediatly and do not shut the truck off. If you wait until it is convenient, like tomorrow morning, that is starting a new drive cycle and you are taking a chance that the PCM will run that test, meet the conditions and pop that code.

my gut is telling me that the guy who sold you the truck knew of the bad cat, cleared the code and sold it hoping that it wouldnt pop on you too soon. This can be tested for with an OBDII scanner. A reset PCM on ALL vehicle since 1996 will show a P1000 code. You might have been able to get the truck for less as most states mandate that the seller is responsible to make the vehicle pass smog... then again he could have told you to kick rocks.

 

Do you know what kind of cats the PO installed? Aftermarket catalytic converter prices are all over the place, OEM being the most expensive and some aftermarket ones being only a couple hundred bucks. If he bought cheapo ones just to pass smog quick or something, I wouldn't be surprised if they didn't last long.

same with the sensors, make sure they are OEM/denso...Toyotas don't do well with aftermarket sensors.

a cracked exhaust before the sensors will definitely trip a code too

 

I had an exhaust leak due to a bad exhaust gasket. I didn't have a code though. I was able to smell exhaust in the engine bay, and searched around enough to see some exhaust smoke after a cold start coming from the manifold to crossover pipe union. I wasn't driving the truck a lot (covid) so I started soaking the bolts in PB blaster for a week before I attempted to take them apart. But I was able to do that without an issue, and installed new gaskets on both the drivers side and passenger side (where the leak was). I have a 1" BL though, so it made reaching through the wheel wells easier, but it was pain to get on some on those bolts. Flexible head closed ratchets are highly suggested.

 

When working on a V6 drivers side exhaust I will take off the brake booster. I have been lucky as everytime I have done the job it has been a non ABS truck. You can leave the brake lines on the brake master and move it forward enough to clear all the studs... LOTS of room then. The 5 minutes it takes to take the booster off is well worth it to avoid the cussing and special tools that you may or may not have.