To test for a plugged catalytic converter, drill a small hole in the exhaust pipe right before the converter, and install a temporary hose fitting. Attach a long rubber hose to it, and end it with a 0 - 10 pound pressure gauge. With the gauge inside the car, drive up a hill or accelerate, and read the pressure. A normal reading is 2 to 3 pounds, but if the flow is restricted you'll read 4 pounds or higher. The drawback to this method is that the small hole will need to be welded shut after the test; a minor job if you have a welder.