I came downstairs to no heat. The control read 70 in room, kicking the setting up to 71 did nothing. I pulled head off the panel to replace batteries, but new batteries didn't help - in fact, now there is no digital readout at all. How do I override the panel for heat? There are 4 wires, as it's A/C as well, red, white, green and yellow. Don't know which 2 wires are to furnace...