A thermostat is a sensor and a set of switches. It reads the room temperature and closes low-voltage circuits that tell the furnace, air conditioner or heat pump to run. When a thermostat seems to stop working, one of three things has usually happened: it lost power, it lost its connection to the equipment, or it is working fine and the equipment is not answering. Telling those apart takes a few minutes.
Why a thermostat stops working suddenly
Dead batteries. Battery-powered thermostats fade, then go blank. Some keep the display alive but stop switching the system on. Fresh batteries once a year avoid most of this.
No power from the HVAC system. Many thermostats, including every wired smart thermostat, run on 24 volts from a transformer inside the furnace or air handler. If the furnace breaker trips or its service switch gets flipped off, the thermostat goes dark too.
A safety switch opened the circuit. On many systems a condensate float switch interrupts the thermostat circuit when the drain backs up. The thermostat may go blank, or simply stop calling for cooling until the water drains.
A blown control fuse. Many furnace control boards have a small fuse on the low-voltage side. A shorted thermostat wire (a nail through the cable, a chewed wire, two bare ends touching) blows it, and everything goes quiet.
Loose or damaged wiring. Wires work loose at the wall plate or at the furnace terminal strip, especially after a thermostat swap or work inside the wall.
Settings, not a failure. A schedule or hold overriding your change, the wrong mode, or a heat pump thermostat set up as if it were controlling a furnace.
Reading the wrong temperature. Sun on the wall, a lamp or TV below it, a draft from a door, or a supply register blowing on it makes the thermostat switch the system off too early or too late. That can look like short cycling; see HVAC short cycling.
Safe checks
- Replace the batteries if it takes them.
- Check the breaker for the furnace or air handler, and the light-style service switch near the unit.
- Look for water in the drain pan or around the indoor unit. A full pan points to a tripped float switch.
- Confirm the mode and setpoint, and clear any hold or schedule.
- Test the link: set the fan to On. If the blower starts, the thermostat is talking to the system, and a no-heat or no-cool problem is in the equipment.
- With power off at the breaker, ease the thermostat off its base and look for a loose wire. Don’t strip or reconnect wires unless you are sure which goes where.
- Brush dust from the vents and sensor area with a soft, dry brush.
Turn it off and call when
- The breaker trips again after one reset.
- You smell burning at the thermostat or the furnace, or see scorch marks on the wall plate.
- Water is spilling over the drain pan.
- You smell gas anywhere in the house: leave first, then call your gas utility or 911 from outside.
What a technician does
We test the circuit from end to end: transformer output, the control board fuse, voltage at each thermostat terminal and the wires in between. If a short blew the fuse, we find the short before replacing the fuse; otherwise the new one blows too. We check the thermostat’s configuration against the equipment (heat pump or furnace, stages, reversing valve) and look at where it is mounted. If the thermostat has failed or does not suit the system, thermostat installation covers the replacement, including adding a common wire for a smart model. If the thermostat checks out, the next stop is the equipment; see furnace not heating.
Older Marin systems
Many older Marin homes heat with a wall furnace or an older gas furnace controlled by a simple two-wire thermostat, sometimes a millivolt one powered by the pilot rather than by a transformer. Those circuits will not run most smart thermostats, so an upgrade means choosing a compatible model rather than doing a quick swap. And a thermostat that keeps the house comfortable on a foggy morning but lets it overheat by mid-afternoon may simply be mounted in the wrong place, near a sunny window or a south-facing wall. Moving it is part of the fix, and a yearly maintenance visit includes a check of its calibration and wiring.