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Discussion Starter · #1 ·
Hi, thank you for taking the time to read this! The truck is a 2015 5.3l bone stock with 41k miles. I don't drive much. I received a CEL and code P0420. I reset it with a scanner and it comes back within 10 minutes of driving. I tried driving on the highway for an hour as well and then resetting since most of my driving is short trips. I have a scan tool from harbour freight and got the below readings. I was hoping someone might be able to help me decipher if its the cats or a sensor?

02B1S1 (v) .080 - .780 fluctuates
02B1S2 (v) .820 steady

02B2S1 (v) .085-.760 fluctuates
02B2S2 (v) .810 steady

I also did the self test option and received :
Catalyst Test Bank 1 catalyst DFCO exit test .091, min .50, max 4.0, STS Failed.
Catalyst Test Bank 2 catalyst DFCO exit test 1.117, min .350, max 4.0, STS OK

With these results would I be able to pinpoint the O2 sensor or if the catalytic converter is bad on bank 1? Ive tried reading through forums with this info, but not successful in finding out, usually just replacing a ton of parts. Thank you for any help.

-Kris
 

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Check the wiring on the rear O2 sensors, the passenger side one can definitely rub along the frame and wear off the wire coating. I just had that happen this past summer and got some O2 codes. I think it may have been grounding out against the frame so I wrapped it in black tape and moved it away from the frame and haven't had a problem since. My truck has 96K miles.
 

· TOTM
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Hi, thank you for taking the time to read this! The truck is a 2015 5.3l bone stock with 41k miles. I don't drive much. I received a CEL and code P0420. I reset it with a scanner and it comes back within 10 minutes of driving.
Kris, unless you have an exhaust leak on that bank between the cylinder head and the catalytic converter, you have a bad cat. O2 sensors have their own trouble codes and your selt test verified this.

If there are no exhaust leaks then it would be nice to know what killed the cat. I would look at the plugs and make sure you don't have a dead cylinder dumping fuel or oil into the exhaust.

Cheap Chicom cats fail very quickly, I replaced mine with the Magnaflow cats and y-pipe 5 years ago and still going strong.
 

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Discussion Starter · #6 ·
Found out today after a little research that the emissions on these trucks are supposed to be 8 years or 80k miles. The lady at the dealership was hesitant, but after looking up the VIN she said that it was true. I would have to pay $130 for the dealer to do a diagnostic check to make sure if its that catalytic converter. I've noticed a bit of a rattle sound coming from the truck after I remote start it on a cold morning. Maybe the cat is moving around inside or pieces broke apart? Thr truck still performs great at low speed and high rpm though.
 

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I know I had the code showing catalytic below efficiency and found that scuffed wire so check the O2 wires. I also decided to run some catalytic converter cleaner through since I was getting the rotten egg smell after longer drives and I haven't had any issues since doing both things. A new y-pipe or converters isn't cheap so check/try things first. With that low of mileage it would be odd to be a converter problem unless there was something running through the engine and out the exhaust.
 

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Discussion Starter · #8 ·
Thank you for the replies, I'm going to check them tomorrow. I liked for leaks today, but everything looked fine and checked the connection from the pcv to the intake. I was surprised at the oil in the plastic lines from the valve covers to the intake tube.
 

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Discussion Starter · #10 ·
I had used a scanner to delete the codes and remove the CEL light, but like clockwork it would come on each morning after about 8 minutes of driving. Yesterday evening I unhooked the negative battery cable for ten minutes and then plugged it back in. This morning when I started the truck and a message came up to roll the driver side window "open then close". This morning after 25 minutes of driving the CEL never came back on. Not sure if this is a different way to remove codes differently than the scanning tool, but so far seems to work. I'm going to look into getting a closed oil/vapor catch can, not sure about the point of one with a vent on it if the oily air from the PCV is supposed to go into the intake, seems like it should be a closed system or your just pulling in fresh air through the vent filter into the air intake.
 

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Discussion Starter · #11 ·
The light came back on again unfortunately. I scheduled an appointment to take it to the dealership so they can use a better scan tool and maybe get some better real time numbers, and prove it is the cats which would be under warranty, but they have to run their $160 scan first. I certainly agree about the oil going through the intake, after reading a lot of posts on this forum, I see many have had to replace cats and wonder if its from the oil or constant switching from 4/8 and heat cycles to the combs.
 

· SilveradoSierra Guru
2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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The PCV system is closed to the air, to limit the amount of unburned hydrocarbons that are vented to the air (as some gets by the rings and valve stems, into the crankcase, and some of the oil in the crankcase becomes airborne), by instead sending them through the engine again to be burned up. However, the downside of this is that some of oil in the air that goes back through the engine, condenses out of the air and builds up in the intake system, particularly on the intake valve, which can cause problems over time.

For a port fuel injection engine, this takes a long time before it becomes a problem, as the fuel being injected also goes over the intake valve, "washing" it a bit. For a direction injection engine (where the fuel is injected directly into the cylinder), no fuel goes over the intake valve, so this buildup happens faster.

I believe your engine uses port injection, so the catch can isn't as necessary, but it does help. They (at least the good ones) are designed to condense oil out of the air and hold it in the can, so much less can build up on the intake valves, and the oil isn't vented to the air, but instead you empty the can regularly to be recycled/disposed of in a more environmentally friendly manner.
 

· SilveradoSierra Guru
2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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As for the code you are getting, it's not a "either the sensor or the cats". The full service manual has a big table, for all the codes the truck has generate, and for each of them, it has a list of things to check, to diagnose what is causing the code to happen. You can google the code, and get generic things to check, and sometimes that's enough to find the problem, but other times the one in the FSM has the extra details you need to find it. And diagnosing the problem can require a bunch of expensive equipment as well.

Alternately, you can google for the problem, and try throwing the same parts other people have replaced in hopes of solving similar problems on their vehicles, in the hope that they also will fix yours. IDK why, but most people like to start with the most expensive parts first (like, say, replacing the cats).
 

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Discussion Starter · #15 ·
Hi, I wanted to let anyone know the end result in case someone searches the same problem. The truck was dropped off at the dealer and they were able to quickly diagnose it was the catalytic converters and will be replacing the whole "Y" pipe even though its the driver side faulty. Crazy thing is that the truck drove fine and no power loss, but only the CEL came on. Even after pulling the two upstream O2 sensors they looked brand new without soot. Thank you for the tips, I did some more chaffing guard on the sensor wires and tried swapping sensors to see if it followed but did not. I also noticed that the STFT was N/A for bank 1 and 99% for bank 2. There was certaintly more things to diagnose with the P0420 code, but the dealerships diagnostic tool was way more real time than my cheap harbor freight code reader. Thank you again for the responses. I just need to find out why it failed after 42k miles of very light duty work.
 

· SilveradoSierra Guru
2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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If the engine wasn't running right for some time, so one bank was running rich, that will also damage the cat. Depending on how bad the problem was, it may not generate a code or run on the CEL.
 
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