When your wall thermostat display screen is completely blank and unresponsive, yet your air conditioning system continues running continuously, it points to a confusing electrical contradiction. This situation typically happens when the thermostat's internal display power source—such as dead batteries or a failed internal display circuit—has failed, but the physical 24-volt cooling relay switch inside the thermostat sub-base remains stuck closed or fused in the active position, sending a continuous call for cooling to the HVAC control board. Alternatively, low-voltage control wiring might be shorted behind the wall. To stop the AC from running, you must immediately turn off the dedicated HVAC circuit breaker at your main electrical panel. You can verify the fault by removing the thermostat faceplate; if the AC stops instantly, the sub-base switch or wiring is faulty.
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