The sensor on the dump pipe isn't the cat temp sensor it is the o2 sensor. Cutting this means your car will run rich on cruising and give you poor fuel efficiency and lower the life of your cat. An error with this sensor will not make the cat light turn on.
The cat temp sensor, which is what activates the light, is plugged into the rear drivers side on the cat. The light being on means it is either grounding when it shouldn't be or your cat has had it.
ok i quess im not gonna cut that off..lols what do u guys think i should do?