When discussing traditional mechanical fluid-filled thermostats or automotive cooling thermostats—though wall thermostats operate electronically via electrical switches—a stuck closed thermostat exhibits distinct operational symptoms. In a heating or cooling context, if an electronic wall thermostat switch becomes permanently fused or stuck closed due to electrical arcing or a short circuit, it sends a continuous, uninterrupted 24-volt electrical signal to your HVAC control board. This causes your furnace, air conditioner, or heat pump to run continuously without ever shutting off, even after your home has far exceeded the target temperature set on the dial. This continuous runtime will quickly overheat your home in winter or freeze your coils in summer. You can verify a stuck closed switch by removing the thermostat faceplate; if the equipment immediately stops running, the thermostat sub-base switch is faulty.
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