0420 code

I got the 0420 code. Just by the number you’d think it would be a fun code but it’s not. I k ow it’s super common and there’s a few threads on it but before I spend the money on parts I want to know how others have fixed it.
Instead of paying $90 a hour for a mechanic to diagnose I’ll fix it myself. Is it most likely a bad Cat (mine has two), a leak, or the O2/AF sensor? I’m guessing the sensors are original.

The cats look welded on, not sure if they are original because I thought they were originally bolt on. My truck has lost a ton of power and MPG recently. About 4 months ago I took the O2/AF sensor out and cleaned them with electrical cleaner which boosted the MPG from 13-19, now I’m back to 14. I’m leaning more towards the sensors but want to know what others think.

 

There is a way to bypass the code with less than $10 in parts. It worked successfully for me and took about 15 minutes.

Take a 1 micro farad capacitor and a 1 watt 1 Megaohm resistor, solder them in parallel, and crimp one end to the positive wire from the o2 sensor, and the other lead to the negative wire. If your sensors are original, then the white wire is the ground and the blue wire is the hot. The two black wires are heater wires. There is a thread on this forum about that but I'm not sure the name of the thread.

That being said, if your truck has lost a lot of power recently, your cat may be clogged up and should be replaced, or if you're like me, just put a pipe in its place and do the capacitor thing.

 

I'm thinking it might be the upstream O2 sensor.

 

More than likely the downstream sensor.. Upstream sensor is usually a P0125 code

 

They are supposed to be bolt on, those are probably some universal aftermarket cats. If the cats on the truck aren't original and were cheap ones then it's probably the cats that are the problem. Cheap cats do not last long because they have very little of the material inside that filters the emissions. O2 sensors pick up on it right away and toss the 0420 code.

Really any aftermarket cat won't last anywhere near as long as OEM ones but OEM ones are expensive, so people usually go cheap with aftermarket.

 

If you don't live somewhere with strict emissions you can either put another set of cheap cats on it or bypass the O2 sensor using an O2 simulator...there are a few tricks to bypass it.

If looking for decent aftermarket cats, go with Magnaflow

There are ways of testing the cats too which is probably wise to do before making any final call one way or the other.

 

Catalytic converters don't go bad, they get killed/destroyed by lean/rich fuel trim issues

If you didn't have any of these issues, it's unlikely your cat is bad and the culprit is most probably an exhaust leak or a bad downstream O2 sensor. A bad upstream sensor would give you a different but specific code

 

This is true...older trucks with high mileage aren't always operating at 100% efficiency and over time either due to lack of maintenance or other factors can lead conditions that might cause cat failure. In my case it was because the PO ran the truck too lean with the supercharger and it fried the cats (sucked). But this obviously wouldn't be the case for someone that isn't boosted

In the case of cheap cats though with little material in them (which sounds like what the op has installed) it wouldn't take much to compromise one of those.

If I were OP is also be looking for exhaust leaks seeing as how the cats have already been replaced at some point...could be they weren't welded in right or there's a crack elsewhere in the exhaust before the sensors

 

I had the upstream o2 sensor go bad on me once and it threw a p0136 code...voltage too high or something like that. Replaced the sensor and all was well. When my front cat went bad due to the lean condition it was a p0420 code

 

I have the same code and the P0171 code. I believe I do have an exhaust leak between the flange before the rear 02. The resistance of the AF sensor was pretty high I believe. That might be the P0171 code for me maybe? I got a new gasket for the flange. Ill see if I can get the rusted bolts off. I think im going to have to get some bolt extractor sockets and a torch :/

 

Could be. I had that code too awhile back (I've dealt with a few codes over the years ) p0171 for me ended up being a small vacuum leak. But I've read that an exhaust leak is a less common but possible cause for it

 

I figured the cats weren’t original. I’m finding more and more hidden problems that the guy I bought it from lied about unfortunately. I read something about a laser temp gun for testing the cats. I’ll have to do that. Unfortunately the guy got rid of the bracket to bolt on new cats so I’ll have to cut and weld new ones in if that’s the case

 

If you crawl underneath and hit the cats with a rubber mallet are they quiet or do they rattle? Rattling from inside means the material is falling apart and that's not good.

 

P0420 means the cat is no longer converting. The down stream wants to see a solid slow fuel trim mix. If it doesn't the code triggers. It doesn't matter if its rich, lean or leaky. If the trim is switching with upstream it's a worn out cat.

 

If I replaced both cats with one would that screw up the system? I thought I read somewhere the Cali regulated trucks had two and others had one?

 

I actually had this code and it was the PCV VALVE,
I replaced the PCV valve , gasket and hose to make sure there were no vacuum leaks and that fixed the code.

 

I would constantly throw the 0420 code on my California emissions Tacoma...The truck ran perfectly...No exhaust leaks, replaced both O2 sensors with new Denso sensors but before I replaced two expensive cats I tried the $4. resistor and capacitor trick. IT WORKED ! NO MORE CODE.

 

If you hav a 2001 then you have 2 cats. All Tacomas after 2000 were built to CA emissions spec

Just look underneath and you'll see them, they aren't hidden. Also you should have a CA 50 state emissions sticker under the hood

If you put one cat on when you're supposed to have two then you'll fail a smog test, if you live in an area that does a visual inspection anyway.

As far as only running one cat throwing a code, I'm not sure...

 

Code 0420

 

https://www.youtube.com/watch?v=Y_ljfnd9H2M