Huge loud sound after engine starts

Hello Folks,

Today , after I started engine, huge loud sound continues with strong gas smell. I have to turn off engine immediately. Before that, there is a piston kind of part dropped out of the hood.

I’m a dummy for car, and hope folks here can give me some diagnosis what’s the problem.

Happy thanksgiving!

That is part of your exhaust system. I’d guess your catalytic convertor has been stolen.


Thanks. How can I confirm it? The door is still locked . It doesn’t seems there is break into the car

Look underneath, see anything missing in the exhaust system? No need to break into the car to steal a catalytic converter.

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Since you know nothing about cars, looking under the car will tell you nothing.

Starting the car should produce a loud noise. Pressing the gas pedal will make it louder. That is your proof this happened. The car will run, you can drive it, windows down, to an exhaust shop for repairs. You probably will see a yellow check engine light but that is OK. But you can’t keep driving it like this.

Thank you guys.

I looked underneath the car. Yes the exhaust pipe is cut and there are other parts left under. This is 2005 car with 200K + miles. I’ll donate it to charity.

There are two pretty new cars just next the old one on my driveway . Just wondering why thief did steal the same parts from the new car

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Just guessing it was quickest and easiest. Your insurance may cover repairs,

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It’s your business…but I wouldn’t get rid of the car just on account of the catalytic converter needing replacement.

Admittedly I’ve not priced one in a while…but I suspect a new converter is far less than the value of your car. Just my two cents.


Is the cat covered under comprehensive? It’s not collision. Not sure how deduc is applied with theft. Vs a tree branch. Moving is collision. Comp is everything else?

Just guessing, but if the wangtie has insurance that would cover the catalytic converter, a police report would need to be filed.
Quite possible, if not a California car, a muffler shop could install a new cat for less than the deductible.


Seeing they unbolted the one end, they may have needed that one for themselves

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I think spring looks more like a valve spring/broke valve vs a piston. Yes, I know it’s a exhaust bolt.

I hang my head. While I’m sitting here pondering what the heck that could be, you folks immediately nailed it. I better go back to bed. Now had there been a sound clip I would have known, having a cat rust off myself.

Had a split catalytic converter, bender guy welded it back together while replacing exhaust, 10 bucks for the weld. Whole cost $120, maybe 15 years ago, lifetime muffler warranty, used it but had to pay $2.60 for a clamp. I think those were the good old days. Whole job cheaper than I could buy parts. No problem passing emissions test.

Yeah welded once, replaced second time for a little over $100 from Rock.