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P0420 Code

6K views 13 replies 7 participants last post by  AVConsult 
#1 · (Edited)
Replaced the rear O2 sensor Friday for a P0136 code, also had the P420 at the time and reset the codes. Drove it and today this one popped on:

Any one had a problem with this one and where did you find the solution?

Air leak in exhaust before rear 02 sensor,

AF sensor error

Fuel System Fault

Catalytic Converter

I guess I am asking for some trouble shooting on this one.

Could the upstream O2 sensor be bad also?
Thanks.

04 Base model 4 cylinder Highlander
 
#3 ·
Funman is quite right.

If you're worried about the front O2 sensor you'll need to watch it on a live scan, is it naturally going lean and rich (not just in closed loop, but is the car really warmed up too?).

You can test the front O2 sensor by creating a vacuum leak and seeing if it goes lean and the car responds. To test that it can go rich you can snap the throttle or put propane in the intake.

If a catalytic converter is going bad, the front Catalyst may not be getting hot enough for example. When it does, it drives the rear O2 sensor rich because it stores up O2 to burn the fuel well in the 2nd rear part of the catalyst.

Exhaust leaks can easily fake a P0420 on the 8th generation, I'm not sure if the 7th generation was setup to be too sensitive too, so be sure to check for those.

For example while the car is cold quickly feel around the exhaust when you first start the engine...it does get hot fast! You'd be looking for a leak prior or at the catalytic converter.

Basically a P0420 should set when the rear O2 sensor is seeing a lot of switching from rich to lean, you know, back and forth, but on the 8th generation, even though the rear O2 sensor is reading pretty steady, which should indicate a good cat, just because it goes rich or lean, across .45 volts too much in a short period, it will throw the code.

I had an exhaust leak after a new cat (I'm a new welder) and even though the cat looked great, due to a bit of switching the code got thrown for example.

I'll try to post up a link to a long conversation I had about P0420 with another member who got a lot of detail out of me ....can't blame him really . . . he really wanted to do all he could before replacing that converter.


...and you really should, even if you determine fast that you need a new one, is it just old, or do you have another condition that's making your catalytic converter go bad? Poor spark? Imbalanced injectors? It's been months and I'm still trying to get my injectors nice and balanced, hard on cheap aftermarket injectors that I could afford.

Here's that promised link:
http://www.toyotanation.com/forum/131-8th-generation-1998-2002/858114-catalytic-converter-4.html
 
#4 ·
If you have a whistling sound it could be a vacuum leak. If you have an ear for vacuum leaks that could be something else to have your engine run a lot of extra fuel and help your catalytic converter go bad. Seafoam destroys catalytic converters; too much and very regular fuel injector treatments instead of rare/occasional use can be a problem too.

Apologies for the 8th generation Corolla comments, I somehow missed you have a 1st generation Highlander, but this advice does generally apply across catalytic converters for a multitude of vehicles.
 
#6 · (Edited)
That is the best article ever. I suppose if you live in an area that doesn't care about emissions and you just want your check engine light off so you know when a new problem comes up, then it's a valid option, otherwise it's an illegal modification of the car (by federal law of the U.S. it's ALWAYS illegal, but truthfully, only local government actually determines legality based on their chosen style of inspections. The Federal Government can't truly dictate what clean air standards really are, local governments just chose to follow it to get Federal funding. How they get to those levels is up to them.)

Saw a youtube video once about adding a resistor to slow down the wave of the rear O2 sensor. Now take my next comment very precisely or you'll misunderstand. The only reason our computers come up with the P0420 code, is because the rear O2 sensor oscillated rich and lean too often. That sensor shows on a scan tool a lot more information yes, but the PCM only sees rich or lean, above or below .45 volts.

A truly bad cat will have the rear O2 mirror microseconds later what the front O2 sensor is showing, but our PCM's are MUCH less intelligently programmed.

True diagnosis would show that there are many causes of this too quick oscillation, but that is all the PCM is looking for. If one understands all the terms in the amazing article shared, then you know all the reasons why and what to diagnose and fix if needed to ensure catalytic converters are only installed when necessary.
 
#13 ·
I'm a bit dubious about cleaning cats...if the substrate is used up, it's used up. Cats get hot enough to burn off contaminants on their own. I'd have to dig em up, may do this at some point, but watching videos courtesy ScannerDanner and EricTheCarGuy as well as a few others, I came to an opposite conclusion, that cleaning cats is probably not effective, or a short-term fix for an underlying problem.
 
#14 ·
I watch Scotty Kilmer videos involving chemicals with one eye half shut; his home brew cocktails probably don't include a whole lot of research as to the contaminants involved and their effectiveness. Lacquer thinner is very potent ; I certainly wouldn't run it through my fuel system at a 1:9 ratio or stronger (i.e. a 1/2 reading on the gas gauge is often less than half capacity). I'd be more tempted to remove a catalytic and soak it in lacquer thinner or acetone as a last ditch attempt to revive it.

And...unless the ceramic substrate actually sloughs off the catalyst, it doesn't "wear out" per say, but rather contaminants coat the catalyst and prevent oxygen storage from what I understand. Removing a portion of the contaminants may indeed push the catalyst efficiency above the monitor threshold for code 0420. Whether that remedy lasts 500 miles or 50,000 miles is probably not something Scotty methodically tracks.
 
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