So I recently got the dreaded P0420 (Catalytic Inefficiency Bank1) code popping up on my ’05 TL with 130k miles. It’s the only code showing up but before I attempt to getting around to replacing the cat I’d like to figure out why it is showing bad in the first place.

I measured some OBD readings and my O2 sensors appear to be fine. They typically bounce between 0.3V and 0.8V with the back sensor jumping slower than the other. My A/F ratio looks good at 14.7. My fuel trim I’m really not sure about. At idle, Bank 1 is usually around 3-4% and Bank 2 is about 6-7%. I took some measurements this morning cruising at about 2400rpm and the LTFT was about 3-4% and short term would flip anywhere from -4 up to almost +20. I’m really not sure what normal is supposed to be as I’m far from an expert here. I’ve linked the graph of what my fuel trim looks like below.

I have tried running some Techron and resetting the CEL but it just ended up popping back on. I did about a month ago have my #5 spark plug fly out of my engine and I did drive a few miles home with it out. Is is possible somehow that ended up ruining my cat and would it have taken a month later and no other errors to finaly have the CEL come on? I’m not too keen on using any kind of defouler because I have emmissions test I have to pass although luckily I just had mine done so I’m good for another 11 months.

https://imgur.com/a/1eMvm

 

Mod edit:

Fixed title

 

If you use a defouler, I'm almost certain you'll still pass the emissions test (unless there is a visual).

 

Replace it

 

TN has only the OBD II and gas cap test from what I can see, so if no codes, the vehicle will pass.
From my experience, using a "defouler" will not usually pass the emissions tailpipe test, especially if the cat is gone, but it can eliminate the CEL light, so it will pass.

 

I guess my main concern is what actually caused the cat to fail. If it was the blown out spark plug somehow them it makes sense to just replace it but I don't want to replace it if there is some lingering problem that's just gonna burn up the next one. Also I can't for the life of me figure out what the OEM cat part number is.

 

P0420 is bank 1, firewall side, downstream cat.
18160-RCA-S00 Converter

 

Perfect! So have these things gone down in price a ton the past few years? Didn't they used to be about $700 in parts alone and not $200?

 

Sorry, wrong cat listed. Guess I had a lapse of memory.
Wife and I have season tickets to the local AA Yankee affiliate and in the 3rd inning realized that I had the wrong cat listed.
I listed the main cat and no sensors detect that unit.
The P0420 monitors the cat on bank 1 and it would be the rear primary converter, not the main converter I listed previously.

Aftermarket units, front right @ $180/$266.

 

Rear primary cat 18290-RCA-L00

 

Ah thanks. Yea the 18290-RCA-L00 is one of the part numbers I found.

 

Well maybe the cat’s fine after all. It originally came on and I tried to run a bottle of Techron Total Fuel System Cleaner (black bottle) in it with some 93 Mobil gas and reset the CEL but the light just came back on in 60 miles so I figured something was actually toast. Figured I’d do one more shot of Techron, the grey bottle just for fuel injectors with some 93 Shell V Power gas. Drove about 30 miles home last night. Turned the car on this morning and no CEL at all. I didn’t even reset it so maybe I’m in luck here.

 

you're band-aiding the real problem.
the code will show back up once it realizes you tricked it

i'm catless and I get an occaisional CEL but goes away on its on. havent looked at the CEL, but I KNOW it has to do with the o2 sensors not reading correctly..i do have a defouler on it

 

But is the cat the real problem or something else?

 

oh! you essientially troubleshooted!
running cleaners through, you made the CEL go away! the sensors noticed cleaner exhaust gasses.
Now, if you REALLY wanted to know...PULL the cat and inspect the substrate

 

Is that even necessary? If it really is the cat then, like you said, the CEL should come back on. No need to take the time pulling the cat to inspect it. At that point I might as well just replace it. My biggest issue was whether or not the cat was actually bad or if there was another problem (vacuum leak, bad o2 sensors, etc.). If the CEL goes off just from running some cleaners through it then maybe it’s safe to assume everything else was fine and if it comes back on then the cat is the only issue?

 

this was a good read!
The Misleading Nature of the PO42O Code - Eastern Manufacturing

 

Yep I read that article. One of the reasons I was trying look at my fuel trim data. My LTFT always seems to be fine yet my STFT seems to stay pretty high. I can’t seem to pick up a sensor for by downstream o2 sensors to check those but my upstream o2 sensors appear fine too. I just don’t see how my CEL would go off by running cleaners if my o2 sensors were the problem.