Car is over heating and heater only blows hot air while driving?

I have an 03 Nissan sentra And
recently the car has started over
heating. But since I've had the car (4
months now) the heater only blows
hot air while I'm pressing on the gas.
I have replaces the thermostat, twice
now because we thought the initial
one was faulty. But it is still over
heating after driving only for a short
distance. The coolant is completely
full. My check engine light came on
and presented a code that said this
Catalyst system efficiently below
threshold
Probable cause:
Large vacuum leak
Fuel system fault
Ignition system fault
Failed bank 1 catalytic converter


Now my car has been starting
roughly for awhile but I thought it
was the battery. And it drives
smoothly although it seems I'm not
getting the mpg as I'm supposed to.
I'm averaging 25 when a sentra gets
30-36

Thank you...