Thermostat Not Turning On the AC or Heat: How to Fix It
The thermostat display is blank, unresponsive, or the system doesn't turn on no matter what temperature is set.
DIY Cost
$0–$10
Typical Pro Cost
$150–$300
Tools Needed
- Small screwdriver
- Fresh AA or AAA batteries (if applicable)
- Soft brush or compressed air
Parts Needed
- Replacement batteries
⚠️ Safety Warnings
- Thermostat wiring is low-voltage and generally safe to handle, but always turn off the system's breaker before disconnecting or reconnecting wires at the furnace/air handler end.
- Never bypass a tripped breaker for the HVAC system by resetting it repeatedly — a breaker that won't hold points to an electrical fault, not the thermostat.
Step-by-Step Instructions
Check for a blank display first
A completely blank screen is most often dead batteries (on battery-powered models) or a tripped breaker/blown fuse for the system — not a thermostat failure.
Replace the batteries
Even hardwired thermostats often have backup batteries. Pop off the faceplate and swap in fresh batteries, then wait a minute for it to reboot.
Check the breaker and furnace/AC switch
Confirm the breaker for the HVAC system hasn't tripped, and that any dedicated on/off switch near the furnace or air handler (often looks like a light switch) is in the 'on' position — this is easy to bump accidentally.
Check the furnace's own fuse or reset button
Many furnaces/air handlers have a small reset button or an internal fuse that protects the system's control board. If the thermostat has power but the system won't respond, check your furnace's manual for this.
Gently clean the thermostat's contacts
On older mechanical (non-digital) thermostats, dust on the internal contacts can prevent it from calling for heat/cool. Remove the cover and gently clean with a soft brush — don't use liquid cleaners.
Check smart thermostat wiring and app settings
For smart thermostats, confirm it's connected to Wi-Fi and not stuck in a schedule/away mode, and that the wires are still firmly seated in their labeled terminals (they can work loose, especially the common 'C' wire).
When to Call a Professional
If batteries, the breaker, and the furnace switch are all fine but the thermostat still won't power the system, or if you suspect a wiring issue at the furnace/air handler, call an HVAC technician — miswired low-voltage thermostat wires can trip a control board fuse repeatedly if reconnected wrong.