Thermostat not working: dead screen, wrong temps, no response

What to do, in order

1. Dead screen: replace batteries (even 'powered' models use them for backup), then check the furnace/air-handler breaker — it feeds the 24V transformer.
2. Screen alive but no response: pull the faceplate and check wire connections (R, C, W, Y, G terminals — look for corroded or loose screws).
3. Reads wrong temperature: it's near a heat source (sun, supply register, kitchen) or the sensor's fouled — relocate is cheap and permanent.
4. System runs constantly or not at all: try the 5-minute test — set heat 5° up (or cool 5° down) and listen for the click at the unit. No click = thermostat or transformer; click + no run = the equipment.
5. Old mercury/bimetal units: replace them — a $100 programmable pays for itself in a season of setback scheduling.

Red flags — stop and call a pro

• Burning smell at the thermostat — wiring short, kill the breaker and call an electrician
• No 24V at the unit after thermostat swap — transformer may have blown; that's a fuse-or-replacement repair

Prevention

Fresh batteries yearly with the smoke-detector ritual, and dust the inside gently when you swap them.

This guide is maintained by the ee.ltd editorial team — reviewed for 2026. Browse all home repair guides or compare hvac costs.

Related guides

• AC running but not cooling: 6 checks before you call
• Furnace short cycling (starts and stops every few minutes)
• AC evaporator coil frozen: why it happens and the thaw protocol
• AC won't turn on: what to check before you call

What this repair typically costs

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• AC replacement cost guide
• Furnace replacement cost guide
• Heat pump installation cost guide