Thermostat Not Working? Start Here

A thermostat is the low-voltage brain that tells your system when to run. 'Not working' usually shows up one of three ways: a blank screen, a screen that's on but won't make the system start, or a smart thermostat that keeps dropping offline. Each points at a different, and fixable, cause.

• Batteries. Many thermostats run on AA batteries, and a blank screen is often nothing more than dead batteries.
• Mode and setpoint. Confirm it's on COOL or HEAT with the temperature set past the current room reading, and that it isn't stuck on a HOLD or an old schedule.
• The condensate pan. If your air handler is in the attic, a full secondary drain pan can trip a float safety that cuts power to the thermostat (more on that below).

If power and settings check out, the cause is usually behind the wall or at the equipment: a blown low-voltage fuse on the control board (often from a shorted wire), a tripped condensate float switch, or, on a smart thermostat, a missing C-wire, the dedicated wire that gives Wi-Fi and variable-speed-system thermostats steady power. Diagnosing low-voltage wiring and control boards safely is a technician's job; poking at 24-volt wiring is how a working fuse becomes a blown one.

In a humid San Antonio summer your AC pulls gallons of water off the evaporator coil every day, and all of it drains out the condensate line. When that line clogs, the float switch does its job and shuts the system down, which often reads at the wall as a 'dead' thermostat. It isn't a thermostat failure at all; it's a safety doing exactly what it should, and clearing the drain brings it right back.

Replace the batteries and check the mode first, that solves a surprising number of 'dead' thermostats for free. If the screen stays blank, the system won't respond, or you're chasing related warm-air or no-heat symptoms, our flat $85 diagnostic (credited to the repair) gets a real tech on the low-voltage side safely.