Thermostat Not Working — Troubleshooting Guide

Your thermostat is the brain of your HVAC system. When it stops working, nothing else does either. Most thermostat problems are simple fixes.

Check #1: Is It Getting Power?

Battery-powered thermostats:

• Replace the batteries (AA or AAA typically)
• Even if the display looks fine, weak batteries cause issues
• Replace annually as a rule

Hardwired thermostats:

• Check the circuit breaker
• Look for a furnace switch that’s been turned off
• Some have a battery backup — replace that too

Check #2: Settings

Sound obvious, but verify:

• Mode: HEAT when you want heat, COOL when you want cool (not OFF or FAN)
• Temperature: Set 3-5 degrees above (for heat) or below (for cool) current room temp
• Schedule: Smart thermostats may have schedules overriding your manual setting
• Fan: AUTO is usually correct; ON runs constantly even without heating/cooling

Check #3: The Display

Blank display:

• Dead batteries (most common)
• Tripped breaker
• Blown fuse at furnace
• Loose wiring

Display on but system doesn’t respond:

• Thermostat may be working but system has its own problem
• Check that furnace/AC unit is getting power
• Check for error codes on the HVAC unit itself

Check #4: Location Issues

Thermostats can give false readings if:

• In direct sunlight
• Near a heat source (lamp, TV, appliance)
• In a drafty area
• Behind furniture that blocks airflow

The system may not run because the thermostat thinks the room is already at temperature.

Check #5: Wiring (Advanced)

If you’re comfortable:

1. Turn off power at the breaker
2. Remove the thermostat faceplate
3. Check that all wires are connected and not loose
4. Look for corrosion or damage
5. Reattach any loose wires
6. Restore power and test

Common wires:

• R (red): Power
• W (white): Heat
• Y (yellow): Cooling
• G (green): Fan
• C (blue or other): Common (power return)

When to Replace

If the thermostat is old, upgrading might be easier than troubleshooting:

Basic programmable: $25-75
Smart thermostat: $100-300+

Smart thermostats can save money through better scheduling and can alert you to HVAC problems.

When to Call a Pro

• Wiring looks damaged or confusing
• You’ve tried everything and it still won’t work
• System has power but thermostat can’t control it
• Installing a new thermostat with unfamiliar wiring

Cost: $100-200 for diagnosis, $150-400 for thermostat replacement with installation.

The Bottom Line

Thermostat not working? Replace the batteries first — that fixes it surprisingly often. Check settings, make sure it has power, and check for loose wires. If all that fails, a new thermostat is usually under $100 and installs in 30 minutes.