Thermostat blank or blinking? Start here

You walk by the thermostat and the screen is blank, frozen, or flashing some message you've never seen. Don't panic — the thermostat is rarely the actual problem. Work down this list in order.

1. Batteries (if it takes them)

Many thermostats run on AA or AAA batteries even when they're also wired to the system. Pop the cover off, swap in fresh batteries, give it 30 seconds. If the display lights up and stays on, you're done.

2. Furnace switch on the wall near the unit

That red switch in the basement or utility closet kills power to the furnace AND the thermostat (most thermostats are powered by the air handler's transformer). If it's off — pets, kids, a broom handle — flip it back on.

3. Tripped breaker

Check your panel. The breaker labeled "furnace" or "air handler" should be firmly in the ON position, in line with its neighbors. A tripped breaker sits slightly off-center.

4. The condensate float switch (this is the sneaky one)

Modern AC systems have a small float switch on the condensate drain. If the drain clogs and water backs up, the switch shuts off the 24V signal that powers the thermostat — which makes the thermostat go completely dark. If your thermostat went blank during cooling season and you find water in the drain pan, that's almost certainly your problem. We can clear the drain and the system comes right back.

5. Blown low-voltage fuse on the control board

If the system has been short-cycling or had a wiring issue, the small 3A or 5A automotive-style fuse on the furnace control board can blow. It's a $1 part, but you have to know it's there. If you're handy, take a look. If not, that's our call.

When to call us

If batteries, breaker, switch, and a glance at the drain didn't solve it, you're into territory that needs a meter and a wiring diagram. We charge a flat diagnostic fee for these calls and almost always have it sorted within an hour.