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Subary impreza that sounds like a sports car

I have a 1998 Subaru Impreza and two nights ago at the end of a long drive my car started roaring. It sounded like the muffler was broken, except the noise was coming from the front of the car, right under the driver’s seat. It sounded like a sports car, or a Harley. Other than the noise, though, nothing else seemed to be wrong. It was driving fine, I didn’t smell any exhaust and the check engine light wasn’t on. I brought it to a mechanic and they said the problem is the catalytic converter and that it needed to be replaced. I’m just a little confused as to why the check engine light wouldn’t be on. I had really thought it was just a bad gasket or maybe the heat shield. Any thoughts?

Loose heat shields rattle, they don’t roar. What you describe sounds like a leaking exhaust system.

If you have a hole in the exhaust system the leaking part must be replaced, even if it’s the catalytic converter.

Did you do any diagnosis to come up with your bad gasket theory?

The light won’t come on for an exhaust leak. It should be fairly easy to stick your head under the car, running at idle, to see if it’s leaking in the vicinity of the catalytic converter. If you can’t do it yourself, seek a second opinion from another shop. Since cats are more heavily built than the rest of the exhaust system, I’m guessing they told you it was the cat because it’s the highest profit item under there.

Get another opinion, but you definately have a significant exhaust leak. A pipe is broken and usually that happens at a connecting point and perhaps this one is at either the front or back of the catalytic converter. Either the metal of an exhaust pipe is rusted out, or the metal housing of the cat is rusted out, or even both. The inside of the cat might be fine and functioning, but the metal case of the cat could be rusted to heck. If it is the original then it is 13 years old.

This is going to be an easy diagnosis all you need to do is get under the car, follow the noise and you’ll find the broken pipe where you see and hear exhaust coming out.

Thanks for the responses. As soon as I get the car back from the shop later today I’ll check and see if I can find the source of under the car. If the metal case of the cat is rusted out would that mean the whole thing would need to be replaced?

Thanks for the responses. As soon as I get the car back from the shop I’ll check and see if I can find the source of the leak. If the metal housing of the cat was rusted out but internally the cat was still functioning, would I still need to replace the whole thing?

Yes, but often it is the pipes connected the cat the rust out faster. Have the shop put the car up on the lift and show you the area where the break is. You should be able to see if the cat is rusted or intact.

If it is rusted you have to replace the whole thing. It only comes as a sealed unit from the manufacturer.

I brought my car to another repair shop today - turns out the problem isn’t the catalytic converter but the flange at the rear of the cat, which rusted so bad is completely fell off. The auto mechanic I spoke with told me to fix the problem I had two options. Either I could replace the cat for around $400-$600, or I could have him weld the exhaust pipe to the cat for about $150. The only problem with that, he told me, is if the cat broke in the future I’d have to replace both it AND the exhaust pipe behind it. Might there be a way to fix the flange that doesn’t involve welding it in such a way that if the cat broke I’d have to replace it and the exhaust pipe? If not, I’m wondering if it might just make sense to go ahead and replace the cat now. If I’m just going to have to do it down the road I’d rather not pay for the cheap fix now and then the expensive fix later.
Thanks for all your help with this, by the way. I’m really eager to learn about this stuff and it’s so great to be able to communicate with people who know a lot more about it than I do.

He’s being honest with you. And the price is fair. Go for it. A job done correctly is always the best approach in the long run.

Yup, I’d sure take that $150 bet. You don’t plan to own that car forever do you?