How-to guide
Thermostat Clicking but AC Not Turning On: Causes and Checks
By Ismail Farooq · Updated 2026-10-01
When a thermostat clicks, it is usually a small relay inside closing the circuit that calls for cooling or heat. So a click with nothing happening afterwards tells you something useful: the thermostat probably did its job, and the call is getting lost somewhere between the wall and the equipment. This guide follows that call, one stop at a time, until you find where it stops.
Does the click prove anything?
On most thermostats with relays, yes: the click is the relay connecting R to Y (cooling) or R to W (heat). Some smart thermostats play a click as a sound effect when you change the temperature (Nest has a “Click sound” setting, for example), and on those the sound doesn’t prove a relay closed. Either way, the checks below work the same.
How a cooling call travels
- Thermostat. In Cool mode, set below room temperature, it connects R to Y (and to G for the indoor fan).
- Furnace or air handler. The thermostat cable ends on the control board there. The board runs the indoor fan and passes the Y signal on.
- Low-voltage wire outside. A thin two-wire cable runs from the board’s Y and C terminals to the outdoor unit.
- Contactor. In the outdoor unit, that 24 V signal pulls in a contactor, a heavy-duty switch that sends 240 V to the compressor and the outdoor fan.
- Compressor and fan. A capacitor gives both motors the kick they need to start.
Each step can stop the call. The trick is working out which one, using what you can see and hear.
Step 1: rule out the normal delays
- Compressor protection. Thermostats wait a few minutes, often about five, before restarting the compressor after it last ran or after power comes back. Many show it: a flashing “Cool On” on Honeywell models, or a “Wait” message. Some outdoor units have their own delay as well.
- Settings. Check the mode is Cool (not Heat, Auto with a wide range, or Off), and the setpoint is at least two or three degrees below the room temperature.
- A new smart thermostat. If the problem started after an install, check that the app knows you have cooling and that a wire was detected on Y. See settings to check after installing.
Give it ten minutes. If nothing has started, move on.
Step 2: what is running and what isn’t?
| What you notice | Most likely cause | Where to look |
|---|---|---|
| Nothing runs, indoors or out | No 24 V at the furnace, a tripped condensate float switch, or the call isn’t reaching the board | Step 3 |
| Indoor fan runs, outdoor unit silent | Outdoor breaker or disconnect off, a broken wire outside, or a contactor that won’t pull in | Step 4 |
| Outdoor unit hums but the fan doesn’t spin | Failed capacitor or a stuck fan motor | Step 5 |
| Outdoor unit buzzes or chatters on and off | Thermostat stealing power without a C wire, or low voltage at the contactor | Step 5 |
| Everything runs but the air isn’t cold | Not a thermostat problem: frozen coil, dirty filter, or refrigerant | Step 6 |
Step 3: nothing runs at all
- Furnace power. Check the breaker for the furnace or air handler and its service switch, which often looks like a light switch on or near the unit.
- Door switch. Most furnaces have a safety switch that cuts power when the front panel is off or loose. Push the panel firmly into place.
- Control board fuse. A small 3 or 5 A fuse on the board blows when a thermostat wire touches something it shouldn’t. A battery-powered thermostat keeps its screen on with a blown fuse, so it can click and show “Cool On” while nothing runs. A thermostat powered by a C wire usually goes blank instead. If the fuse is blown, find the cause before replacing it with the same rating.
- Condensate float switch. In summer this is one of the most common causes. Air conditioners make water, and a clogged drain makes it back up. A float switch in the drain pan or drain line then cuts the cooling call (or the 24 V power) so the pan doesn’t overflow. If there is water in the pan under the indoor coil, or a small switch on the drain pipe has lifted, clear the drain line. The AC usually starts again once the water level drops.
- Is the call reaching the board? With power on and cooling called, measure from Y to C at the furnace board with a multimeter set to AC volts. About 24 V means the thermostat and the cable in the wall are fine. No voltage means the problem is at the thermostat: a loose Y wire, a broken wire in the wall, or a setting. Our multimeter guide shows each test.
Step 4: indoor fan runs, outdoor unit doesn’t
The thermostat and furnace are working, so the call is being lost on its way outside.
- Breaker. The outdoor unit has its own double-pole (240 V) breaker in the main panel, often labeled AC, condenser or heat pump. If it has tripped, reset it once. If it trips again, leave it off and call a technician.
- Disconnect. Next to the outdoor unit is a small box with a pull-out block or a lever. Make sure it is in the on position. Some contain fuses that can blow.
- The low-voltage wire outside. The thin cable running to the outdoor unit is easy to damage with a trimmer or lawnmower, and rodents chew it. Walk its length and look for cuts. A broken wire here gives you 24 V at the furnace board and nothing outside.
- Listen at the outdoor unit while someone sets the thermostat to cool. A single solid clunk with nothing after it means the contactor pulled in but the motors didn’t start. No sound at all means the 24 V call isn’t reaching the contactor, or the contactor coil has failed.
Beyond these checks, the parts involved are behind the outdoor unit’s service panel, with 240 V and a charged capacitor. That is the point to call an HVAC technician. Telling them what you have already ruled out saves time.
Step 5: humming, buzzing or chattering outside
- Hum but no fan. The most common cause is a failed run capacitor. It is a cheap part, but it stores a dangerous charge, so have a technician replace it. Turn the unit off at the disconnect in the meantime: a compressor that hums without starting overheats.
- Rapid chattering or buzzing at the contactor. The contactor is pulling in and dropping out. A common cause after installing a smart thermostat is power stealing: without a C wire, the thermostat draws power through Y and the contactor can’t hold. Adding a C wire or the brand’s adapter fixes it. See no C wire options.
- Starts, runs a few seconds, stops. Often a pressure or temperature safety switch tripping. Some outdoor units then lock out until power is cycled. That needs a technician to find the cause.
Step 6: it runs but the air isn’t cold
If the outdoor fan spins and the compressor runs, the thermostat and wiring have done their part. Replace a dirty filter first. If the indoor coil or the large copper pipe at the outdoor unit is covered in ice, turn cooling off and run the fan alone for a few hours to thaw it, then call a technician if it ices again; that usually points to low refrigerant or poor airflow. On a heat pump, warm air in cooling mode usually means the O/B setting is reversed (see O/B explained).
Thermostat clicking but no heat
The same approach works for heating. The click is the thermostat connecting R to W, and you follow the call into the furnace.
- Check W reaches the board. With heat called, measure W to C at the furnace board. No voltage means the problem is the thermostat, its settings or the wire.
- Watch the start-up sequence. A modern gas furnace starts the small draft inducer fan first, then a pressure switch confirms the vent is clear, then the ignitor glows or sparks, the gas valve opens and the burners light. The main blower comes on about a minute later. Where it stops tells you a lot:
- Nothing happens at all: furnace power, the door switch or the board fuse (as in Step 3).
- The inducer runs but nothing lights: often the pressure switch, a blocked vent or intake, or on high-efficiency furnaces a blocked condensate drain.
- Burners light for a few seconds then go out: very often a dirty flame sensor.
- Read the LED code. Most furnace boards have a light that blinks a fault code. Count the blinks through the small window in the panel and match them to the chart on the inside of the furnace door.
- Heat pumps heat with the same Y wire as cooling, so a heat pump that clicks but doesn’t heat follows Steps 3 to 5 above. Also check the thermostat is set up as a heat pump, not a furnace. See heat pump thermostat wiring.
- Boilers often use a zone valve that opens before the boiler fires. If the valve motor runs but the boiler doesn’t, the valve’s end switch or the boiler itself is the next place to look.
If you smell gas, don’t try to restart anything: leave the house and call your gas company from outside.
When to call a technician
Call a licensed HVAC technician for anything inside the outdoor unit, a breaker or fuse that trips again after one reset, a capacitor or contactor replacement, a compressor that hums without starting, refrigerant or icing problems, and furnace fault codes you can’t clear by cleaning the filter or drain. The checks in this guide narrow the problem down so the visit is shorter. For other symptoms after a thermostat install, see the troubleshooting checklist, or tick your wires in the wiring tool to confirm each one is on the right terminal.