If it rattles (I am pretty sure it is the rattle I am having at about 2750 RPM consistently) do I need to replace it? I understand this porcelain(?) inside can clog up my exhaust system. Also I have an annual e check around here. Should I just go ahead and replace it? Thanks

Check out Autozone.com for the cats. They have a few choices all under $100.

FCP Groton has good prices on cat converters. Mine was $116 for a Bosal cat.
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Volvo 240 Links | Volvo 240 Racing Gallery

foster hit the nail on the head.

If you do have to remove the cat I had good luck with this procedure.

I took the flange apart then cut the pipe with a tailpipe cutter just behind the cat. This leaves you some leverage if you need to wiggle the pipe back and forth. I installed an aftermarket clamp on the pipe and tapped each side of the clamp alternately to draw the pipe out(off?) of the next pipe.

FYI - A tailpipe cutter is a large tubing cutter that has 4 cutting wheels so that you only have to swing it just over 90 degrees to cut a pipe. Sears offers one. I have used mine 4 or 5 times and love it!
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2 8s & 2 7s 600,000 miles total

If it rattles, first be sure it's not the heat shields around the cat or the exhaust pipe rattling.

If the heat shields are gone (as they are on my '86 245) then a rattling cat (as does my 245) surly means that the ceramic honeycomb is starting to fracture.

One symptom my car sometimes has is extreme loss of power as the ceramic chunks rearrange themselves to block a speedy exit of exhaust gases.

Hit a few potholes, and they'll bounce around, rearrange themselves again, and then you'll have a free flowing (or freer flowing) exhaust again. I have a new cat to install next month -- before safety inspection.

Expect to pay around $150, more or less, for an aftermarket replacement cat. Don't forget a new gasket, bolts, and clamp.
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Don Foster (near Cape Cod, MA)