I have a 01 Accord ex 4 cyc that is just burning through gas. I know I have a CEL for a bad cat. Can a bad cat cause a car to get lousy mileage? I thought a o2 could but just having a bad cat causes this? It is not clogged just had inside parts broke away and you can hear them rattling around inside it at times.

 

If the catalyst has broken apart then it is most likely impeding exhaust flow. This will cause the engine to work harder to push the exhaust out, and you probably are getting some goofy MAP readings from backpressure and reduced volumetric efficiency.
If you cannot afford a new catalyst at the moment, take out the old one, and remove the broken parts. You will have a P0420 code, but the engine will at least be running correctly.

*remove the broken catalyst internals and reinstall the catalyst shell.*