The temp range the BSCU can deal with is 0 to 1000 degrees C.
This is down to the output voltage of the Brake Temp Monitoring Units on each gear, they amplify the output voltage of each brake temp sensor to a voltage from 1 to 9V.
The BSCU takes 1v as 0 deg C and 9v as 1000 deg C , at 0 deg C and below there will be no discernable output from the temp sensors so the BSCU defaults to 0.
Any voltage out of this range caused by a short circuit or probe failure will show XX on the lower ECAM DU.