I have several Trane units. One thermostat has no display, but appears to be still working. Low Battery?? I've cmpared a functioning and a non-functioning thermostat. There appears to be a soldered in battery?? Battery on functioning unit reads .4 VDC, non- functioning unit reads 0. Is this likely the cause of no screen display? If so, are the batteries readily available? (The soldering is no problem for me.)