Will AC run if the thermostat is not working?

Whether an air conditioning system will run when the thermostat is not working depends entirely on the nature of the thermostat failure and how power or control signals are interrupted. If the thermostat suffers a complete internal electrical failure, a dead screen from exhausted batteries, or a blown low-voltage fuse on the indoor furnace control board, it loses its ability to send 24-volt control signals to the HVAC unit, causing the air conditioner to remain completely dormant and unresponsive. However, if the thermostat display goes blank due to a dead screen while its internal cooling relay switch remains stuck closed or fused in the active position, the AC can actually continue running continuously without shutting off, posing a risk of freezing your evaporator coils. To stop or start the system manually when a thermostat fails, you can check the circuit breakers or test low-voltage wire connections. You can verify the thermostat's status when manual wire bridging successfully triggers operation.