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Does this sound like a bad catalytic convertor?

Okay, my wife has a '99 Toyota Sienna with over 200 k miles. Recently, it has began making a loud exhaust noise. I looked at the exhaust system and cannot see any obvious holes. Also, her check engine light has been on for a while. I checked it and it said misfire in cylinder 5 (can’t remember the code). Changed plugs and all, still the light. It also idles very high. There’s no rotten egg smell that I can tell, but she does say that it seems like it doesn’t have a lot of power when accelerating. I was just wanting advice before I take it somewhere. Thanks.

We don’t have enough information

Is it still missing, after you replaced the plugs?

A shop could hook up the scanner and see if the misfire is still present. They can also monitor all the oxygen sensor signals, to give them an idea of the cat’s condition

But a backpressure test and a 5 gas analyzer will give them even more information

It’s also quite possible you have an exhaust leak way up front, possibly from the exhaust manifold gasket, or the exhaust manifold flange gasket

that will make noise, and cost you lots of power

Yes, it was still missing after changing the plugs. I put some seafoam in it and it cleared the check engine light. But after a while it will come back on.

Have the codes read (free at most parts stores) and post the codes back here…Also, does it have an ignition distributor or does each plug or pair of plugs have it’s own coil?

You need to scan it again. Coil or wire can cause misfire. This is the 6 cylinder, where 1 coil feeds 2 plugs. Did you replace the wires? Did you track down the exhaust leak?