Thermostat Clicks but Nothing Happens? Go Listen at the Unit

The Click Is Good News

A click when the thermostat turns the system on or off is normal. Lennox says it is often the sound of the relay switching inside the thermostat. So a click with nothing running means the thermostat has power and is asking for heating or cooling; something between the wall and the equipment is not answering.

If the screen is blank or the thermostat restarts instead, that is a power fault: see blank or black thermostat screen.

What to Check Before You Call

Six things, none of which need a panel opened:

1. Mode and setpoint. Carrier’s first check is that the thermostat is set to “Cool” and the temperature is set lower than the current room temperature (the reverse for heat).
2. Give it five minutes. Thermostats hold the compressor off after it last ran. Honeywell says the delay can be up to five minutes under normal operating conditions. See Delayed Start and Honeywell Wait.
3. The breakers. Check the breakers for both the indoor and the outdoor unit. Reset a tripped one once only: Bryant says that if it trips again after being reset, leave it off and call a professional.
4. The outdoor disconnect. Carrier notes that the outdoor unit usually has a disconnect box nearby; check it has not been switched off or pulled out. Look only: Bryant says not to remove the disconnect cover.
5. The furnace door. Honeywell’s checks include the furnace door kill switch. A panel that is not seated keeps the system off.
6. The condensate drain. Bryant explains that when the drain backs up, a safety float switch can then interrupt the cooling call. Water in the drain pan confirms it.

Then Listen at the Equipment

Set the thermostat so the system should start, then stand by the outdoor unit for cooling, or the furnace or air handler for heating.

A hum or a fan that will not start is a reason to switch the system off at the thermostat and call, not to keep trying.

Contactor and Capacitor: What They Are, and Why Not to Touch Them

These two parts sit inside the outdoor unit and account for many “clicks but will not start” calls.

• The contactor is a switch the thermostat controls. Bryant describes it: the thermostat sends a low-voltage signal and the contactor closes to supply high-voltage power to the compressor and fan motor. It can fail from electrical arcing that burns and pits the contacts, or from insects and debris in the mechanism.
• The capacitor stores energy to start the motors and keep them running. Signs a technician looks for, according to Trane: bulging, cracks, and oozing liquid.

Neither is a homeowner repair. Carrier warns that capacitors store high-voltage electricity that can cause serious injury or death, even when the power is disconnected, and Trane says they should only be tested, discharged or replaced by a trained professional. Bryant tells owners not to open electrical access panels at all.

When It Is the Thermostat After All

Sometimes the click is real but the call goes to the wrong place, usually after a new thermostat was fitted. The sign is that one function works and another does not: heat runs but cooling never starts, or the fan runs but nothing else.

Check that each wire is in the terminal the thermostat expects. Google’s wiring guide gives the usual roles: in most systems Y controls cooling, W controls heating and G controls the fan. It also warns not to rely on wire colour alone. With the power off at the breaker, compare the terminals with the photo of your old thermostat’s wiring.

A Meter for the Thermostat End

If you are comfortable working at the thermostat with the breaker off, a meter shows whether it is getting power at all. Leave the outdoor unit’s panel to a technician.

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When to Call a Technician

• A breaker trips again after one reset.
• The outdoor unit hums, buzzes or makes other unusual noises.
• The system has frozen over, or you suspect a refrigerant leak.
• The basic checks above do not bring it back.
• Anything that would mean opening an electrical panel, or touching a capacitor or contactor.

Frequently Asked Questions

My thermostat clicks but nothing turns on. What does that mean?

The click is usually the relay inside the thermostat switching, so the thermostat is working and asking for heating or cooling. The fault is between the wall and the equipment: check the breakers, the outdoor disconnect, the furnace door and the condensate drain, and allow five minutes for the compressor delay.

Why does my outdoor unit hum but not start?

Carrier says a humming sound from the outdoor unit usually means a motor is trying to start without the boost it needs from the capacitor. Switch the system off and call a technician.

Can I replace an AC capacitor myself?

Manufacturers say no. Carrier warns that capacitors store high-voltage electricity that can cause serious injury or death even with the power disconnected, and Trane says only a trained professional should test, discharge or replace one.

The indoor fan runs but the outdoor unit does not. Why?

Carrier lists a faulty capacitor, a bad contactor or no power to the outdoor unit, and notes it can also be a safety lockout. Check the outdoor breaker and that the disconnect is on, then call.

How long should I wait after the click?

Up to five minutes. Honeywell says its compressor-protection delay can last that long under normal conditions.

Related Guides

• Smart thermostat black or blank screen — when the thermostat itself has no power
• AC short cycling — when it starts and stops
• Heat pump not turning on — the heat pump version of this checklist
• Smart thermostat troubleshooting — every fix guide by symptom

Sources

• Lennox: is a thermostat supposed to make a clicking sound?
• Carrier: AC unit not turning on
• Carrier: What is an AC capacitor?
• Bryant: AC not turning on
• Bryant: AC fan not spinning
• Trane: heat pump capacitors and signs of trouble
• Honeywell Home support: Wait, Waiting for Equipment, or flashing Cool On
• Honeywell Home support: blank display on an RTH6360
• Google Nest Help: identify a thermostat wire