P0420 (catalyst efficiency below threshold) code keeps coming back!

Guinnessmike · March 19, 2021

I have a 2016 Sierra 1500 with the 5.3L engine, 111k miles. About 6 months ago I changed the spark plugs & wires and one of the wires accidently got pulled out by the stupid long bolt in the steering shaft that run way to close to one of the spark plug wires. The truck misfired but I was only a few miles from home. I limped it back, realized the problem, plugged the wire in and all seemed good. Unfortunately, 10 miles later the check engine light came on again, this time just showing P0420 code (catalyst efficiency below threshold, Bank 1). I changed all (4) of the O2 sensors, but the code came back after 10 miles. I then bit the bullet and changed the drivers side cat with a aftermarket weld-on from rockauto, nope. I then paid a muffler shop to replace the passengers side cat (generic) and fix my sad welding on the drivers side. The light stayed off for about 50 miles this time but as always came back on. I then tried to put a spacer between the 2 rear O2 sensors to pull them out of the exhaust stream, light on after 10 miles. I even tried plugging the old 2 rear o2 sensors in the wiring harness and just zip tying them to the frame so they were breathing fresh oxygen, but still the light! My mechanic is out of ideas and suggesting taking it to the stealership and having them replace both cats again with the OEM setup for @$2000. I have almost zero faith that will solve the problem and that it will just be more money that I don't have going into the trash, and the dealer will just shrug their shoulders. Any other suggestions of things to look at/try before jumping off that cliff? I am a backyard mechanic with just a basic OBD2 reader. I do know that all (4) O2 sensors are putting out voltage, but really don't know how to interpret that data.

durandetto · March 19, 2021

If you don't have any exhaust leaks and you're using good fuel it's most likely the cats. Aftermarket cats are a lot cheaper for a reason. I currently have my y-pipe sitting on the floor of my barn, just replaced it for a kooks off-road y-pipe. I'd be willing to part with it for a few hundred plus the cost of shipping.

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MaverickZ71 · March 19, 2021

Beware of fake knockoff parts made in China. You can chase yourself in circles trying to fix something with crap parts.

Also, keep in mind that today's diagnostic systems are so damned sensitive/picky that even if you have a not-so-bad-part from brand other than GM or AC Delco, the GM system may still not like it.

CamGTP · March 19, 2021

A tune would be cheaper than new cats too.

jim_55379 · May 19, 2022

Yes Mike I have the same code P0420 once in a while will also read P1153.

when I first purchased the vehicle 2008 chevy with 5.3 would get a CE light about once every 2 or 3 months now it comes on every week or 2. I purchased a Blue reader still can't figure out what the data means or the issues they don't seem to help me at all. I replaced all plugs and wires about a year ago and had a shop install 4 new Denso O2 sensors. No change. Lately light is coming on more frequently. Every time I come to a light I cringe waiting for the idle to change a bit then light to come on. Only issue is a slight roughness to idle once in a while. I took to Midas they said Cats are good and nothing wrong with vehicle. I have no rotten egg smell and they did an intake injection cleaner for the tune of $ 450.00 (robbery which I could have probably done myself). light came on about a week later. these computer vehicles are the pits.

Edited May 19, 2022 by jim_55379