2003 Tacoma P0420 CEL

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Still working on old junk. My truck has recently started to throw a consistent 2 trip check engine light, in addition to the periodic evap CEL.
Inspection expires end of November. The evap CEL is so periodic that I can clear codes and get all systems to a ready state except evap which is always the last to clear, so 1 not ready = a pass for NY OBD2 tests.

Did the after-cat exhaust on the truck a couple months ago. P0420 research has led me to believe it might be an exhaust leak before the 2nd o2 sensor. Climbed under the truck hoping to find a loose bolt right after the cat but before 02 sensor but no such luck. Second step was to start the truck and check for a leak. Very slightly exhaust leak right after the cat. A bit frustrating as I used all new gaskets and hardware on everything I changed. The flange after the cat is a bit worn away from rust over the years., so i can't really tighten the bolts down anymore, its already flange on flange. The last time I had the exhaust change for muffler I took it to a small shop that reused all the old gaskets and cross threaded things, but never had a P0420 with the orange permatex they used.

I stopped at Napa this morning and got myself a new exhaust gasket, and also some high temp sensor safe permatex. The hardware is all new so it should be pretty simple to take out the 2 bolts, drop the pipe and inspect the current gasket. I might just apply a healthy coating of permatex and send it, or maybe switch out to the new gasket. I am pretty sure the new gasket I put in there was a cone shaped type, where as the one Napa gave me is the flat type. Just need to get it sealed up enough to clear the codes and drive for a couple days to pass OBD2 inspection.

Anyone ever deal with one of these codes? Hoping its not going to turn into o2 sensors or an actual issue with the cat.
p0420 is 110% going to be a bank one converter , back in the early obd2a and obd1 days it was meh maybe oxygen sensor or exhaust leak etc but now anything 02+ is pretty spot on as far as CEL codes. it would have a oxygen sensor code if bad oxygen sensor and if it was a exhaust leak pre cat it would show a bank 1 lean code. how the ecu is measuring the converter is pre and post cat oxygen sensors, if bank 1 sensor 1 (front of converter) is reading X amount and the ecu isnt reading a big enough lowered difference by the post converter sensor 2 then it triggers the P0420/430 code.. assuming because its a 03 it probably has higher mileage and probably is on oem converters soo its 99.9% going to be the converter, if the code isnt constant like you said there is probably some platinum left and a oxygen sensor defouler kit will more than likely go unnoticed and make you pass
You would put that defouler on bank one sensor 2 (post cat) , it just makes the ecu read a lower carbon rating
Also if you had a exhaust leak.. it would be obvious and youd hear it , idk why people think converters last forever and the ecu is telling you it needs a converter
p0420 is 110% going to be a bank one converter
thanks for the information but I think you might have some misunderstandings about the situation.
truck is 2003 2.7 4cyl 4wd, but only has 95k miles.
the exhaust leak was very small, but you could feel the hot air blowing out when you put your hand around the exhaust flange. when i said it was intermittent code, i meant intermittent CEL. the code would always show, but would need to pop at least 2x before it threw a CEL. CEL would come on every 2nd day give or take.
the leak is post cat, but pre-secondary o2 sensor.
I'll attach a picture so you can sort of see what i am talking about. to the right is the cat, then the flange which had a leak, then the secondary o2 sensor. I researched it quite a bit online and this seemed like the cheapest simplest, especially considering the only thing that changed recently of the variables was me replacing the cat-back exhaust.

results. the bosal gasket that i had used (rockauto) was pretty crushed up and appeared to be smoked. i used a beefier gasket with metal interior ring and layers of exterior material. i also applied a rather generous helping of permatex.
the cat flange is shot to the point that the metal around the bolts pulls in towards the midpipe flange. probably a temporary fix, just hoping to clear inspection for now. for the first time in my life i actually read the permatex directions and followed them. 1/8" to 1/4" bead, hand tighten until sealant starts to push out, then wait an hour before final tightening. applied a coating to both sides of the gasket. i let it sit overnight so a good 12+ hours to cure, but not the full 24 they advise.
3 starts and 20 miles of driving and now all systems are in a state of ready except the evap. the first 2 starts were very short, one to check for hot air leaks, the second to stop at the bank half a mile from home. the 3rd trip was the 20 mile commute to work. after 10 miles i had no codes but catalyst was still NOT READY. by the time i got to the office (20 miles) the catalyst was READY. scheduled inspection for tomorrow morning.


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you know a exhaust leak POST CAT would help you right because it significatly lowers the O2 sensor readings from PRE-POST converter... im telling you $100 its going to need a converter and 93k is perfectly in range of needing it I had to have a warranty cat put on my vette at 39k miles... also they are only rated at a 10yr lifespan for the platinum inside them , not trying to be negative but its going to need a cat
Im telling you the defouler is way to go and p0420 is bank 1 front cat, downstream converters arent monitored
The tacoma doesn't use a thread in o2 sensor.
I've looked into the sensor extenders and defoulers, same type of product, different name.
Haven't found one for the tacoma 2 bolt o2 sensors, except for a "test pipe" o2 sensor extender for about $80 on ebay.
Throw up a link if you find one.
you dont have to find one... just tack weld the o2 sensor to the defouler bung and drill hole and weld the defouler to the pipe , or buy a like $0.25c 02 bung and weld it on and use a different oxygen sensor they are all same 4 wire
I ran defoulers on all my mitsus and hollowed out the cats until I replaced the header and exhaust completely. No CELs
Did the same on my last Mazda, worked perfectly for a few years til I sold it
Only noticable difference was a slightly stinky exhaust and a few less mpg
Status update. 65+ miles driver.
Still no codes, no check engine light.
The truck passed OBD2 inspection, all systems ready except EVAP.
Gonna let it ride, see what happens.


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~250 miles, finally popped another CEL.
Finally had a day in the upper 40's in NY so the EVAP system finally set and tripped the P0446.
Catalyst system still holding strong.

Haven't had a chance to try out my new ebay smoke machine to see what might be throwing the evap.
More likely than not its the charcoal canister, gas tank vent, or a broken or cracked line somewhere.
Status update on this thread.
Ended up taking the truck to a couple dealers that low balled me for trade in, offering $5k of less.
Ended up craigslist'ing it for $9k and ended up selling it to a local kid for $7k.
Ran into the kid a couple days ago and his grin was still ear to ear. I recognized the truck by the small dents on the roof line where a limb had fallen on the truck, dropped by the previous owner.
Extremely happy with the truck and its still running great for him. He took it away to college and is back for the summer.

Had a slight exhaust leak, but he didn't say anything about any CELs, so thats good news.
Managed to crack the rear diff somehow? But we didn't get into the how, just that he's waiting for some parts to come in.

Always nice to see someone else getting good use out of your old vehicle, and not a pissed off buyer.