I am trying to trouble shoot a motor issue I am having.

Motor: Reedy Sonic 7.5
Issue: Wont work with the sensor cable plugged in. But will work without the cable plugged in. I tried 4 different sensor cable, same issue. The esc runs other motors fine sensored.

My first guess is that the sensor board is shot. I was running the motor around 160 deg and when it died it was only 140.

Probably the hall sensors. Replace the board
http://www.teamassociated.com/reedy/parts/details/959/

what are the causes for the board to fail? I would like to prevent/reduce this from happening again.

The sensors can fail for a number of reasons, and there isnt really much you can do about it unfortunately. Luckily they are cheap enough to keep some spares around.