Simple Ways to Test for a Clogged Converter
Of the techs I run across, most just remove an oxygen sensor and let'er rip. The mechanics I grew up with always dropped the exhaust. They made it look so easy too.
My favorite method is to use the Thexton back pressure test kit.
This inexpensive kit consists of a special air hammer bit (to punch a hole) and simple threaded nipple that screws into the exhaust pipe and adapts to an ordinary vacuum/pressure gauge. Self tapping screws are included so you can plug the holes after your done testing. I prefer the torch or welder to plug the holes when I'm done.
With the back pressure adapter and pressure gauge installed, run the engine at 2000 - 2500 RPM. Normal exhaust back pressure will measure between .5 and 1.5 PSI. Anything over 2 psi. will likely cause noticeable driveability problems.
1.) Invest in a good back pressure test kit. You can buy the Thexton kit like I use, or an adapter that screws in place of an upstream O2 sensor
2.) If you use a set-up that's similar to what's shown in the photo above, you know how hot the test fitting can become. If this is a problem, try using an old section of Silicone DPFE sensor hose at the fitting. DPFE hose is better suited at handling heat than regular vacuum hose.
It's never a good idea to run an engine with the oxygen sensor removed. Hot exhaust gasses and possibly flames will be shooting out of the hole which was previously plugged by the sensor.
Wear hearing and eye protection when operating an air hammer.
Use caution around hot Exhaust pipes.