If you had just changed the heads I would look at replacing the cable to the black head, or swapping it with one of the others. The printer gets the head temp from the black one. I cant see any reason changing the heads would have damaged the platen thermistor. I was getting a temperature too low error once after a head change, and it was down to a bad black head cable.
Also try turning on in service mode, there is a thermistor check feature on the menu that will show if it is getting a signal