P0420 Code

SteveTR6 · Post by **SteveTR6** » Wed Feb 16, 2011 6:50 am

I had an earlier thread on this, which mysteriously disappeared. So I guess I need to start again; please bear with me.
I have a 2002 Astro with 139k on it, and it's started giving me a P0420 code. At first I thought it would be the fault of a leak at the flange between the cat and the muffler, but I fixed that Monday (finally warm enough to get under it and drill holes!). I reset the code, and yesterday took a long (45 minutes each way) drive with the reader hooked up and running live diagnostics. About 40 minutes in, the cat test finally ran and the code immediately popped up again.
So my question is this: The cat was replaced by Midas less than 2 years ago and is under warranty. Around the same time, I replaced the downstream O2 sensor. I'm not suggesting neither could be bad at this point (probably just over 100k on it then), I just don't want to go on a blind part-replacing binge if you know what I mean. It looks like there are at least 2 O2 sensors in that area, right? One in front of the cat and one behind? Am I right on that? The FLAPS has new Bosch sensors for anywhere from 60 to $80 each, and being out of work I don't want to replace one unless I'm pretty sure it's the correct one. Is there a way I can test for function on one of these?
Any Ideas what's wrong?
Thanks
Steve

LiftedAWDAstro · Post by **LiftedAWDAstro** » Wed Feb 16, 2011 10:36 am

As you can see, we are re-arranging the forum a little and it appears your original thread is gone. Oops, sorry about that.

Anyway...I don't know how to test the oxygen sensors. All I know on them is the voltage should be fluctuating between basically 0 and 1 when they are working. Not sure you can see the readings or not but if they are flatlined, the sensor is dead. The P0420 code almost always points to the cat being bad. The O2 sensors have their own codes for being bad.

I know V8famvan posted in your other thread so hopefully he reads this one!

v8famvan · Post by **v8famvan** » Thu Feb 17, 2011 11:37 pm

SteveTR6 wrote:I had an earlier thread on this, which mysteriously disappeared.

Yeah, I was the reason it disappeared - must have hit the delete button for some reason. Sorry about that. #-o

Commonly if a vehicle sets a P0420 & no other codes .... it is a cat convertor. Only in about 1% of the cases that I work on is it caused by something else like an exhaust leak, O2 sensor or faulty injectors. Aftermarket cats are usually significantly cheaper than a factory cat because they do not have the performance ability of a factory cat convertor. Translation: it could be faulty after 2 years. A simple test that can be done to check the operation of the cat convertor is to check the temp before & after the cat with an inexpensive temp sensor/infrared gun. Typically the outlet of the cat convertor should be 100 degrees hotter than the inlet side - indicating the convertor is functioning.

LiftedAWDAstro · Post by **LiftedAWDAstro** » Fri Feb 18, 2011 8:09 am

v8famvan wrote:A simple test that can be done to check the operation of the cat convertor is to check the temp before & after the cat with an inexpensive temp sensor/infrared gun. Typically the outlet of the cat convertor should be 100 degrees hotter than the inlet side - indicating the convertor is functioning.

EXCELLENT tip! =D>

Cobra · Post by **Cobra** » Fri Feb 18, 2011 8:28 am

That truly is an excellent tip i've never heard of that before

SteveTR6 · Post by **SteveTR6** » Fri Feb 18, 2011 6:23 pm

James,
Thanks! That's some awesome advice, and exactly the kind of expertise I was hoping for. I will check the temp thing tomorrow, right before I head to Midas and make them give me a new cat...
Steve

photo_van · Post by **photo_van** » Sat Aug 15, 2015 6:49 am

v8famvan wrote:
SteveTR6 wrote:
Commonly if a vehicle sets a P0420 & no other codes .... it is a cat convertor. Only in about 1% of the cases that I work on is it caused by something else like an exhaust leak, O2 sensor or faulty injectors. Aftermarket cats are usually significantly cheaper than a factory cat because they do not have the performance ability of a factory cat convertor. Translation: it could be faulty after 2 years. A simple test that can be done to check the operation of the cat convertor is to check the temp before & after the cat with an inexpensive temp sensor/infrared gun. Typically the outlet of the cat convertor should be 100 degrees hotter than the inlet side - indicating the convertor is functioning.

Old thread, great tip (are there guys like this still on here??). I've been throwing and re-throwing a PO420 for a year now and the cat is rattling. Been looking at the O2 sensors via the Torque app on the iphone; looks good there. As if that isn't enough of a diagnosis, I'll get under there with the IR gun that my brother inlaw gave me for xmas (been wondering what I was going to do with that)

alwaysstuck · Post by **alwaysstuck** » Sun May 14, 2017 6:35 pm

any help or advice would be greatly appreciated here... I got the cat code p0420 when my friend ran the scanner on it....should I change the downstream 02 sensor first? or shall I just go for an aftermarket cat.converter ? The truck is a work van, ..I just need it to pass inspection in NY and get me back and forth to work, so I can eventually put some more cash into it. I believe the man with the scanner said this is what I have to clear in order to pass.

ive heard a lot of people say that after market parts may not last 2 years or so..but that's ok... as long as it passes, and I can continue to work.

from the way it looks under there, .. I might be able to swap it out with basic tools, and save the money bringing it to a shop.

Is anyone familiar with cleaning these cat. converters ?... laquer thinner in fuel ?, .. soap and water ?.. Is this an option ?

thanks in advance.
MIke

AstroWill · Post by **AstroWill** » Thu May 18, 2017 9:04 am

alwaysstuck wrote:ive heard a lot of people say that after market parts may not last 2 years or so..but that's ok... as long as it passes, and I can continue to work.

The problem is that a lot of people will replace the cat but fail to address what caused the cat to die. Which in turn leads to the new cat meeting an early death.

See: https://sites.google.com/site/astrosafa ... aust/p0420

blacknome · Post by **blacknome** » Wed May 31, 2017 2:07 pm

I got the ghost P0420 code for well over a year. After getting pricing on a direct fit I just replaced mine with 2x magnaflow universal cats. Within a couple days the light came back on pinpointing which o2 sensor was bad.

Funny thing is since I bought the van in 06 ( 11 years ) I always averaged 13.5 - 13.7 mpg. I am now up to 14.3-14.5.

Either the sensor fouled the cat or the cat was worn out at 270 thousand kms / 168 thousand miles.
No complaints either way

Chevy & GMC Vans

Chevy & GMC Vans

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