Upstream or downstream O2?

I had a diagnostic ran on my 96 Tacoma since the check engine light came on and I have three options...catalytic converter, upstream O2 sensor or downstream O2 sensor. None of these are cheap parts so if I could get it right the first time that'd be swell.

Any suggestions on where to start or how to narrow it down? Code was P0420

 

Someone correct me if I'm wrong but the "downstream" sensor is the only one that would set the light off. The "upstream" is just for a baseline.

 

Most likely the 02 sensor. I don't think you have a downstream sensor.

If it's been running fine with no CEl up to recently, than the cat is fine.

 

Yes that's the one

 

That is usually a cat related problem or exhaust leak. I've had that code before and it was my front cat, replaced it and all was well.

When I had a bad O2 sensor it was a different code entirely, P0136 if I recall.

For some reason people never want to believe that their cats might be bad, but with the age and mileage these trucks have nowadays it's entirely possible you need a new cat.

If you don't need to pass any kind of strict emissions testing then you can get an aftermarket cat for cheap. Toyota cats are very expensive. If you do replace the cat then don't go too cheap as the really cheap ones have very little of the filtering material in them and they won't last long.

Magnaflow makes a quality aftermarket option. But I would try and confirm it's the cat(s) before you go replacing them. A crack or leak in your exhaust can also cause this code too

 

Check for exhaust leaks upstream of the O2 sensors before assuming it's the catalytic converter. Ethanol gas over 10% can also throw a P0420.
A good muffler shop can heat test your catalytic converter if you don't have a heat gun.

 

also if you do have 2 sensors start with down stream. I have been struggling with this issue for almost a year. Have changed everything once. Both cats and both sensors. Changed down stream twice. went about 3 weeks CEI came back PO420. Said screw it bought the URD simulator and bing, bang, boom, all is good in the neighbor hood lol. So check for exhaust leaks first. Then put on a computer that can check and scan while running. Go from there. That's my opinion. Also the simulator works off down stream sensor.

 

Also knowing the mileage of the truck would help in diagnosing the trouble.

 

A very common cause for P0420 is exhaust leaks. Even pinhole leaks can trigger a P0420. I would check for this first

Also, catalytic converters don't really go bad, they get ruined by lean or rich fuel trim issues. If you didn't have any of these, your converters are likely to be fine

 

Just curious, but your truck is low mileage for a cat to be replaced.
The cat will plug up from ruining rich.

What was the issue with caused your cat to plugup.

I’m still on my original cat at 620,000

 

It'll plug up running rich or burn out from running lean, was running lean. PO never did any fuel mods for the supercharger so most of the cat damage was done even after I got the 7th injector installed.

Got the dreaded p0420 and confirmed it was a bad front cat, replaced both with CARB approved Magnaflows and been good since

 

It's 218k I believe

 

That codes means the ECM is not seeing enough difference between the front sensor and the rear depending on your emissions package you might have one or two cats .

A tripped code is always the complete circuit never forget.

Sucking in Air between the sensors is always good.

 

Just a thought .....since it’s not going to cost you anymore in the long run, change the front O2 sensor. At 218,000 it will need to be replaced anyway if you must replace Cat.
Or just change the O2 sensor and get the URD device.

 

what are your emission laws like in AK? if you don't need to pass smog, you can ignore it or get the urd simulator.
if you do need to pass smog, the walker direct fit cat isn't too expensive. i think i paid 250 from rockauto for the cali legal one. i replaced all my o2 sensors trying to get better gas mileage and fix p0420 code. the code was only finally fixed by new cat. i think it was my short trips without highway miles that killed the cat. i still have my old one. entertained the idea of keeping it installed primarily and only swapping the new cat on around smog time.

barring an easy fix like an exhaust leak, you'll probably have to replace the cat.

 

depending on shipping, the URD will run you about 127.00

 

If you really are tight on funds just do a search on here or YouTube for the $4.00 resistor and capacitor p0420 fix.
My cats were weak after 130,000 and 20 years of low speed driving and this did the trick.......also replace the front O2 sensor with OEM or genuine Denso if the original is still in there, since it controls the amount of fuel being dumped into the intake.

 

An upstream O2 sensor can trip a code as well though. If it's not getting a proper reading (to set that baseline) the downstream will pick up too much difference and trip the code.

Not saying this is what is going on here. Just saying I'm pretty sure an upstream O2 sensor can also be responsible for a fault code.

 

True! I guess I meant to say the downstream one would be the one that tells if the cat is working or not.