Catalytic converter might be bad

[JTalley]

My van has a lot of the symptoms of a failing cat- funny idle, not much high speed power, odd-smelling exhaust, etc. Also, after driving for a bit (particularly at speeds higher than 40), the check engine light comes on. It used to only do it every now and then, but now it's more frequent and I seem to have less power at higher speeds. If I turn it off and then start it back up again, the light stays off for a while.
The engine runs rather warm- it doesn't take long to heat up, and the temperature gauge usually stays a little bit above halfway.
My gas mileage ranges from around 10-26 mpg. Typically, I'll go easy on the throttle to save gas. That works, and the mileage stays high for a while. Then it gets terrible mileage. I found that when it gets bad like that, if I gun it and do fast accelerations it gets better mileage for a while, then it drops again, and I have to go easy on it again. Repeat. I'm not sure why this is, but I'm pretty sure it has something to do with that cat.

I'm trying to figure out if that is the cat or not before I go cut it off.

[Cobra]

A simple test that can be done to check the operation of the cat convertor is to check the temp before & after the cat with an inexpensive temp sensor/infrared gun. Typically the outlet of the cat convertor should be 100 degrees hotter than the inlet side - indicating the convertor is functioning.

i love how simple a test this is

[JTalley]

Thanks!

Now to find an IR gun... I think O'Reilly's loans them, but I'm not sure.

[CaptSquid]

What color is the inside of the tailpipe? Have you checked the O2 sensor?