2008 Suburban P0420

I have a 2008 Suburban, 5.3 L , 110,000 miles.
I replaced the cats last year, and recently replaced all the O2 sensors.
During inspection the Evap system had trouble resetting from the codes being cleared. After changing the fuel cap, and several days of driving it finally reset. Now the service engine light is on with the repeat code p0420.
I did have it cleared , and it has shown it`s ugly head again.
I have read of several sources of this issue with multiple things to check, and in the process of doing this.
Hoping for some great advice, and or info if someone else has had the same issue I am dealing with. I am having a hard time believing the cats are bad.
Thanks for any reply.

you need a scan tool and post the fuel trims ..

Where can I find the information on the fuel trim readings?
Thanks for replying.

I would be looking at the post cat wave forms, before fuel trims....the trims can be fine and still cause a P0420......

So I would take a graph of the pre and post O2 sensor data....if the post "mirrors" the pre, bad cat.......

Were the cats OEM or aftermarket? If aftermarket, smaller than original? Same with O2 sensors? Was the reason for cat failure found and fixed? Like running rich, misfire, etc.

You can get something like this for around 20 bucks, on Amazon.......there are better ones, but more expensive....it will read and clear codes from your vehicle, read data, plot data of sensors, etc., on your smartphone....will need a third party app, like Torque Pro, which costs about 5 bucks....


https://i.ebayimg.com/00/s/MTYwMFgxNjAw/z/~JQAAOSwEzxYZMNR/$_58.JPG

Thanks for replying.
No the original issue, was not fixed. If there was an original issue. After changing cats( aftermarket) the service light came on about a month later, showed O2 sensors acting up. I am going to order it right now. Hopefully it will help me figure this issue out, again thanks for replying.

One way to check cat efficiency is to check the inlet temp vs outlet temps on the cat, fully warmed up the outlet should be a minimum of 125 or more degrees than the inlet temps. If not the cat is inefficient, this assumes the engine is running correctly.

so the real deal is , you had the converters replaced but the engine was not properly analyzed ..code comes back .. did you do this or a repair shop ?

this requires a scan tool to retrieve diagnostic data to determine why the driver side converter according to the PCM is not properly operating .

now the hack way to do this with NO scan tools is swap the downstrean O2 sensors put left on right right on left see if you get a different code if so , the O2 sensors need be replaced OR wiring issues which is common on the driver side because the tech's are sloppy on wiring down there..

if the code does NOT change then DIAGNOSTICS ARE NEEDED.................

O2 SENSOR WAVEFORMS, FUEL TRIMS,MAF FLOW, ITS A LONG LIST ..

Again thanks for everyone answering my post.
I did get the plug in obd reader, but I am still getting the P0420 popped up in less than 24 hours after clearing it.
My evap system is not resetting, I have replaced the fuel cap, the hose that goes from the fuel neck to the canister.
Any reply and or idea is welcomed.
Thanks in advance.

I said you need a scan tool ............. then post what I stated ..

post all codes also .

Sorry you feel I am ignoring you, but you really don`t have to reply to me like I am your child not making through the door before curfew is up. I am asking for help, not to be scolded like a puppy whom has wet on the floor. I don`t ever in my life, and when my father was alive he demanded me to do something, when I asked for his help, and he helped raise me. I guess I am a different person. If another human being was in a jam and needed some help, and I could help them. I would not scold them for not following the directions I have offered them. Maybe they can`t access what I asked them use, maybe their life is more complex than mine.
Again I do appreciate any of the responses I have received, and I do thank you all. But I do what I can if it isn't exactly what is everyone thinks is right, it may be ok with me. Asking for help isn`t a crime.

The code is P0420.... The bank 1 sensor 1 is all over the place on the graph. Sensor 2 is just a flat line.... Bank 2 Sensor 1 is not as bad as the first bank sensor has more definition to the graph, sensor 2 bank 2 flat line... I have spoken to my local mechanic, and we are leaning towards replacing the bank 1 sensor 1, due to the graph on it.
Thanks again for any reply.

Fond this guy on you tube... Awesome! Schrodingers Box (https://www.youtube.com/channel/UCnIQ-_hRpyHR68f23dQQHUg)

NO ENGINE DATA .. IMO bring to a repair shop much faster , less stress . no need to waste your time here.

my guess , your not the mechanic .. just questioning the repair shops work .

many do ask questions about repairs that shops have done . problem is we here, get no data and this person asking questions , has no idea what the data is ..