Does it always read -15C? or intermittent? If it is intermittent, does it change when the hotend is moving (e.g., drawing a big 200mm diameter circle). Try recording the temps via pronterface or octopi (if you have it), to see if it comes and goes. I've had issues with the wires snapping in the x cable chain and were very intermittent.
I'm also curious exactly where you checked the thermistor resistance? was it at the thermistor or at the board connection?
It would be weird for a thermistor reading to go bad on a board without something else causing it (e.g., a loose wire or something causing a short).
Also, see this post and search some others. Its a more common issue than you probably expected... http://folgerforum.com/t/mintemp-error-thermistor-cable-broken/2456