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Check engine light after muffler gasket replaced

My muffler started to be noisy about a week ago. This morning a mechanic told me I had a hairline crack in the manifold and it would be $600 to replace it. I then took it to my regular mechanic who said that just a gasket needed to be replaced. After they did the repair the car was quiet and running fine. A few hours later the car is not as quiet as when I first got it back and it a little shaky especially when idling. I drove it around a bit and it seemed fine, but the check engine light came on and I think there may have been a vague smell.

Take it back and have the code read. It may or may not be coincidental. Or have the code read at Autozone or the like. Either way, post the code here for a better answer.

Take it back.

It’s a P0420 code. I will take it back, I was just trying to get an idea as I can’t take it back til Monday.

If there is a P0420 code by itself, it almost certainly means that your catalytic converter is bad. In every case of a lone P0420 code I have ever seen, it has been a bad cat. If there are other codes as well it could be a different problem. The leak could have something to do with the code being produced. Have the code cleared and see if it comes back.

Not sure if there are other codes as the guy at Auto Zone only gave me this one. Would the original noise or the gasket replacement relate to this since they just happened? I also have to drive out of town on Wednesday so I don’t have a lot of time to clear the code.

It may be that P0420 often comes from a bad cat - but its also the case that many people get a new cat and still get the P0402. That’s partly b/c one thing that can trigger this code is exhaust leaks. It seems obvious that something went wrong with the manifold gasket repair AND/OR the manifold is cracked. Either way the code should be taken care of when your shop gets it right.

The code was cleared when it was read, and after driving it a bit it came back on.

So may be the case that I do in fact need to replace the manifold? How approx. how much would a manifold replacement and a cat replacement run?