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2012 gmc acadia code 0420

Acadia starts stuttering when driving. Mechanic said it was the catalytic converter but it wasnt worth the time and money to fix. It has 100k miles on it. Could it be the 02 sensors? I believe there is a lower and upper censor. Should I start with that before having to replace the 3way cc?

Every vehicle that I’ve had come into the shop with a P0420 DTC had a bad catalytic converter…


Get a second opinion and if it is the Catalytic Converter start calling muffler shops to see if one has a price you can live with . Also if your state has Emissions test make sure the shop will have your vehicle pass after the work . The shop you just used may not have a good source or saw other problems with your vehicle.

If you live in California the cat could be pricey. Elsewhere, not so bad.

Thanks for the replies. I am in Kentucky btw. Thanks.

We don’t know what the shop did to determine a bad cat. With a 100K I would think the cat lasts much longer. You could give it a shot and replace the upstream sensor. That’s the sensor that most likely set off the code. It’s a long shot, but only ~$40 if you do it yourself. Otherwise I would say follow other suggestions and look at replacing the cat. Shouldn’t be that expensive for this car.

Let me ask a dumb question. The mechanic said I have a 3way cat. What exactly does that mean.


Great info. Very much appreciated sir!

Rockauto sure lists a lot of cats for your Acadia - front left, front right, center, rear…does it have 4?

I have no idea. I’m confused on this bc I’m getting conflicting info

Your mechanic probably hooked up a scanner to monitor the upstream and downstream senors signals.

That’s how I determine if the catalytic converter is bad.


So there is only one cat?

There’s two pre-cats and one main cat.


P0420 is for the right front catalyst. The front converters are the primary converters, they are monitored by the computer, the rear converter is a secondary converter, it is not monitored by the computer.

On this vehicle the rear converter is listed as the three way converter.

Make sure the shop has done the test poster “Tester” mentions above. That’s the best way to determine if the cat is good or bad. If it tests bad you’ll have to replace it if you want the engine to run correctly again. I presume the shop is telling you that it’s better to save your money and live with an engine that doesn’t run correctly. Maybe, maybe not. If you live in a no-emissions testing state, that’s your call.

That was very good information

This could very well be due to a partially plugged catalytic converter(s)

If the “stuttering” is due an exhaust restriction . . . aka catalytic converter . . . replacing oxygen sensors isn’t going to solve anything
Did your mechanic also perform a back pressure test?

I presume he monitored the signals of all of the oxygen sensors and determined that the downstream sensor(s) were switching as rapidly . . . or almost as fast . . . as the upstream sensor(s)

Im not sure what exactly he did but I have recieved such great info on this forum I’m taking it in tomorrow to a different mechanic to see what he says. At least now i have some sort of explanation for him. Thanks

If it’s the front right it is about $150 at Rockauto. That’s where I got mine for my Riviera. Install was just a bolt up. Not a big problem if I remember right. Now on my Pontiac, I got a code and the dealer cleared it. It came back again right away so they replaced one of the cats on warranty. Normally they last a long time unless they rust off but they sell them for a reason.

Let the mechanic diagnose the problem. If he’s good, he will.