The single cat is what I have, replaced the mid muffler. You may be right about a plugged cat but you would really hear an exhaust leak. Is the heat shield still attached to the top of the cat?

I hate to say this but are your floor pans in good shape along with the lower part of your firewall, meaning are there any rust holes in them?

The condition your describe should be somewhat easy to isolate by a process of elimination. The blower is off but is there still heat coming from the ducts, in other words is your monovalve shutting off? If not, you will still get a lot of heat coming from the heater. Is there an exhaust leak, easily detected, start the car, put on the emergency brake and look under it or take it to a shop with a lift to inspect it. Remove your carpets and see if there is any rust up under the dash or under the floor.

Keep us posted

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I really gets fun when they all don't run

84 500SL
99 Jeep XJ
93 Jeep YJ
03 Custom Softail