Thermostat not working or blank screen

Thermostats get power one of two ways. Some run on batteries alone. Others draw low-voltage power from the furnace or air handler through a wire, often called the C wire, and may use batteries only as backup. A blank screen means that power source is gone.

If the screen is on and the system still ignores it, the thermostat is powered and the problem is in its settings, its wiring or the equipment it is trying to control.

• Dead batteries. The screen fades, then goes blank. Some thermostats stop sending calls before the display dies.
• Power off at the equipment. A tripped breaker, the furnace switch turned off, or a furnace door left ajar after a filter change.
• Condensate safety switch. A clogged drain fills the pan and a float switch cuts power to the thermostat so the AC or furnace cannot keep making water.
• Blown low-voltage fuse. A small fuse on the furnace control board protects the thermostat circuit. A shorted wire, often at the outdoor unit, blows it.
• Failed transformer. The part that makes low-voltage power has burned out.
• Loose or damaged thermostat wire.
• Failed thermostat. Age, a power surge or a faulty screen.

• The heat or AC does not start when you change the setting, with no click from the thermostat.
• The system runs constantly and will not shut off.
• The room temperature shown is clearly wrong compared with a separate thermometer.
• Short, frequent cycles.
• Settings or schedules reset on their own.
• Buttons or the touchscreen do not respond.

Before blaming the thermostat, rule out placement. A thermostat in direct sun, above a lamp or beside a supply register reads warm and behaves as if it were faulty.

The technician measures voltage at the thermostat and at the control board. That shows within minutes whether the thermostat, the wire or the equipment is at fault. If a fuse has blown, a good technician finds the short that blew it before installing a new one. Otherwise the new fuse blows too. If the drain tripped a safety switch, the drain is cleared and tested.

When the thermostat itself has failed, replacement is straightforward on most systems. Heat pumps, multi-stage equipment and systems without a C wire need a compatible model and correct setup. The thermostat replacement cost guide covers what goes into the price.

A thermostat fault never means the furnace or AC needs replacing. A fuse that keeps blowing, a transformer that failed or a drain that keeps tripping the safety switch does point to a problem in the equipment that will come back until it is fixed. If the thermostat checks out and the furnace still will not run, continue with the furnace repair guide.