P0420 Buick Code - Catalyst System Low Efficiency Bank 1

what is a ataiyst system low efficiency ,bank 1

You need a new catalytic converter. Age and low octane gas can cause the catalytic convert to fail.

09 buick enclave 3.6 after replacing coil and spark plug on #3 cylinder the engine light keeps coming on and it is now staying on. Po420 is one of the codes its showing. is this a catalyst converter plugged up? the other code that has now added to the list is P0008 Engine position system performance, both are Bank 1

Clogged or damage. Usually, when the spark plugs or coil fails, the raw fuel goes through and ignites with the heat of the catalytic converter, which will damage the catalytic converter.

I replaced the catalytic converter on my 2005 buick lesabre. The code is still there and reads: catalyst system efficiency below threshold bank 1. What else could be causing it?

Did you use new factory Catalytic Converter? How long did it take for the code to comeback?

sorry wrong place

I have the same problem with a 2005 buick lacrosse, the cat has been replaced 5 times and I still get the P0420 code. They also replaced the 02 sensors a couple of fuel injectors wire and plugs and a few other things. They are now stumped. Sometimes the code comes up in 100 miles after replacement and sometimes 200 miles. This last time with a magnaflow cat it lasted 1300 miles. I took it to another garage and they said the fuel trim was high at 17%. They have not replaced it with a factory cat because of the outrageous price of over $1500. The car only had 55,000 miles on it when this code started.

Unfortunately I never had the same problem, but I've checked a national database of repairs for your model year and other than the parts you have replaced, the Intake Plenum Gasket was on the list of repairs. The Intake Plenum Gasket will also cause the the increase on fuel trim. I hope that helps.

Thank you. I will see what this new garage says and go from there. Will post the outcome when they find out.

I have the same code coming up on my 2011 CXL. I got at 97k miles. I changed the spark plugs when I first got the car. And the code started appearing about 1miles ago. I have about 106k miles. I used NGK plugs instead of the factory ones. I thought that could be the problem then I thought about the plenum being the problem after I read this post. It takes about 500 miles before it reappears. I hope its just something simple. I would rather replace the 02 sensors. Can anyone help

Try using high octane gas for a couple of weeks and if possible ad a fuel additive to clean the catalytic converter. Although no additive can repair a failed catalytic converter, sometimes they can clean deposits from a functioning converter and help restore its efficiency, which could stop triggering the P0420 code for you.

I still haven't got mine fixed, The latest update is to replace the O2 sensors with factory OE ones. After that will be an OE converter but the other garages say I should not have to use a 1300 converter and something else is triggering the p0420 code although no one has come up with a solid diagnosis.

I have a 2008 Buick LaCrosse, and the code came up. I've had the catalytic converter replaced, spark plugs changed, O2 sensors changed. Still hasn't helped, and I am very frustrated. Will not be purchasing a Buick ever again.

Did you use factory catalytic converters? any other codes or symptoms?

I just bought a 2017 Buick encore. I have had it since February 19, 2025 been driving it back-and-forth to work no issues about two weeks ago. The check engine light came on and it’s reading. P0240. I can’t seem to find anyone who knows what they are doing . My question is can I use third-party parts because if I use a factory it’s gonna cost me so much money? I put in cat cleaner to fuel as well . One Mechanic wants me to get the extension. If that doesn’t fix the problem then I obviously need a new catalytic converter, correct?