• If you currently own, previously owned or want to own an Avalanche, we welcome you to become a member today. Membership is FREE, register now!

P0420 HELP!!!!

loumazz

New Member
Joined
Jul 24, 2020
Messages
4
I have a 2007 5.3L Avalanche, bought 3 months ago the repair shop has it more than me..uugh
141,700 miles on it, bought as is( my bad)
Day i took it home check engine light came on
P0420 and p0430 code it won't pass inspection so took it to a mechanic 1st thing he did was a full tune up spark plugs ect.. said that can throw the code..
Code came back ,
2nd time in, found a cheap China y pipe,
So put in a New California emissions Y pipe with new cats, 900. And new 02 sensor on bank 2 sensor 2, still p420 code,.
3rd time back New ECM and programed,
Still p420..
4th time back no answer yet..
On the code reader

Bank 2 sensor 2 looks perfect. .6 volts steady...
Than drops to .01 no volts..
Than up again fluctuating
Than 0 volts for awhile...
Than .6 steady perfect
HELP what can it be
New ecm ,new cats , new 02 sensor
Side note:
Bank 1 sensor 2 always .7 volts steady
So he swapped the sensors
The problem stayed on bank 2
 
Last edited:
First welcome to the club and to Avalanche ownership...

P0420 is bank 1, P0430 is bank 2. If you only changed bank 2 sensor 2 O2 sensor then you should change the other 3. They meant to be change at 100,000 miles on the maintenance schedule, your AV has 141,000 and were probably never changed.

The ECU is looking for a difference between the front and rear O2 sensor to show that the converter is doing it's job if either sensor is bad the it can set the code...

So if they haven't been changed them that's the first thing I would do, it's part of routing maintenance so it's so it's not throwing money at it chasing parts it has to be done for it to run correctly.
 
Last edited:
First welcome to the club and to Avalanche ownership...

P0420 is bank 1, P0430 is bank 2. If you only changed bank 2 sensor 2 O2 sensor then you should change the other 3. They meant to be change at 100,000 miles on the maintenance schedule, your AV has 141,000 and were probably never changed.

The ECU is looking for a difference between the front and rear O2 sensor to show that the converter is doing it's job if either sensor is bad the it can set the code...

So if they haven't been changed then that's the first thing I would do, it's part of routing maintenance so it's so it's not throwing money at it chasing parts it has to be done for it to run correctly.
Thanks,
The mechanic switched the sensors i not sure if he switched them back, the problem stayed on bank 2
I should have him just put all new ones in at this point...could it be a broken wire???
The problem seams intermittent
 
Thanks,
The mechanic switched the sensors i not sure if he switched them back, the problem stayed on bank 2
I should have him just put all new ones in at this point...could it be a broken wire???
The problem seams intermittent

You have 4 O2 Sensors 2 Upstream 2 downstream. The mechanic should be able to easily read what data they're throwing back using their OBD2 reader to monitor exhaust.


Have the mechanic replace all 4 at the same time and save the headache (1A Auto has them as a set for ~$150.). (If you try to do them you're going to strip the Sensor nut guaranteed).
 
You have 4 O2 Sensors 2 Upstream 2 downstream. The mechanic should be able to easily read what data they're throwing back using their OBD2 reader to monitor exhaust.


Have the mechanic replace all 4 at the same time and save the headache (1A Auto has them as a set for ~$150.). (If you try to do them you're going to strip the Sensor nut guaranteed).
 
P0420 Diagnostic Chart as attachment below.

Static O2 sensor voltages mean nothing. Have to plot them to help diagnose a P0420 or P0430

This is in Car Diagnostic Pro on my Android phone. I like it because it will plot 2 at a time.

This is normal O2 sensor operation after 2000 rpm for >1 min and then let idle. Upstream O2 sensor is suppose to cross 0.45V (which means it is controlling). If downstream O2 sensor crosses 0.45V or cycles significantly at same frequency then you have a problem as catalytic converter is not working right.

If you can't do it yourself, then find someone better to work on your vehicle. Current guy is just using the S.W.A.G. system of diagnosis.

Screenshot_20201002-163125.png
Screenshot_20201002-163232.png
 

Attachments

  • 2004AvalancheP0420.pdf
    43.5 KB · Views: 5
Last edited:
Another thing you an do is to create a vacuum leak and look at the response of the O2 sensors. Here I pulled off the vacuum line to the brake booster and held my finger over it enough that it would stay running. (This was on a Blazer with 2 upstream and only one downstream sensor). This definitively checks the cat converter as you can see more than 1 second between upstream and downstream sensor response. This is the kind of thing the PCM actually does to test to cat converters, and if it doesn't like the time between responses - then it sets the code as a P0420 or p0430 cat problem.

Hope this helps get rid of some myths.

Screenshot_20190324-173608.png
 
Last edited:
Another thing you an do is to create a vacuum leak and look at the response of the O2 sensors. Here I pulled off the vacuum line to the brake booster and held my finger over it enough that it would stay running. (This was on a Blazer with 2 upstream and only one downstream sensor). This definitively checks the cat converter as you can see more than 1 second between upstream and downstream sensor response. This is the kind of thing the PCM actually does to test to cat converters, and if it doesn't like the time between responses - then it sets the code as a P0420 or p0430 cat problem.

Hope this helps get rid of some myths.
Is the theory here that the time difference should be as short as possible, otherwise the CAT is clogged? That's the only thing I can think that would cause an excessive delay in the rear O2 output. What would be the pass/fail expectation for response time?

The 2005 I just bought last weekend and fixed threw a P0430 code today for the first time. It was running +25 LTFT on that same bank, until I replaced the O2. I'm wondering if the super rich condition, for an unknown amount of time, could have fouled the cat. Also wondering if a very rich induction service may clean it at all.

Good info!

Thanks!
 
Last edited:
time difference is the proof that the cat has capacity and is doing it's job. super rich condition will definitely foul a cat converter, as will a lean misfire.
 
Last edited:
Anyone had a universal replacement cat welded in?

Alternatively, anyone know the lowest cost option for a new set of cats that won't continue to set a code?
 
You can change the filters here to input your engine vin code (8th digit) and year of Avalanche.

https://www.summitracing.com/search/part-type/catalytic-converters/product-line/ap-exhaust-universal-catalytic-converters/make/chevrolet/engine-size/5-3l-325/engine-family/chevy-small-block-gen-iii-iv-ls-based-engines?N=engine-type:v8

Is your pre-cat o2 sensor operation verified to be correct by voltages swinging up and down across 0.45V at both idle and 2000 rpm? Don't put a new cat on unless it is. If it runs rich or misfires again, it will ruin the new cats. What do you have for an OBD2 scanner?

Unless you have to pass emissions for some reason or the vehicle doesn't run well, you have time to diagnose properly.
 
Last edited:
Hey guys, I am seeing P0420 as well. Would you be able to help me diagnose the issues if I can post data and graphs?
We'll give it a shot! Quick question though, how much oil do you burn between oil changes? Have you ever upgraded the drivers side rocker/valve cover (per TSB https://static.nhtsa.gov/odi/tsbs/2012/SB-10062524-7690.pdf) that reduces PCV oil consumption? I ask because years of burning oil can accelerate catalytic converters death, which can cause P0420...
 
Last edited:
I just got the Avalanche last Saturday, so I don't know exactly what has been done. It doesn't seem like it from the Carfax. BTW I was looking at the diagnostic procedure and was wondering what "HP2" is.
 
I just got the Avalanche last Saturday, so I don't know exactly what has been done. It doesn't seem like it from the Carfax. BTW I was looking at the diagnostic procedure and was wondering what "HP2" is.
Need more context, how/why is it referring to HP2?

Tahoe/Suburban RPO code HP2 means Hybrid motor...
 
I had a 0420 error on mine about 2 years ago but everything was functioning properly and there were no red flag readings. I unhooked the negative battery terminal overnight and then reattached the following morning. It went away and has not come back on since.
 
I had a 0420 error on my 2007 LTZ, replaced the Cat, but after 8 months re-appeared. Replaced the one O2 sensor, but still produced a code, I returned it to the muffler shop that installed the Cat. Upon inspection, I had an exhaust leak due to the driver side front and rear exhaust manifold bolts had snapped off in the head causing the exhaust leak. After repairing the exhaust leak, problem was solved, engine light no longer on.
 
Back
Top