Google Nest Thermostat E74 Error is due to lost of power to the red wire. I will show you how to troubleshoot this problem. Mine turned out to be from a blown HVAC fuse. I will show you how to replace the HVAC fuse. 0:00 Intro 0:16 E74 Error 0:25 How dose the red wire work 1:14 Diagnosis 1: Tripped circuit breaker 1:22 Diagnosis 2: Red wire not connected correctly 1:43 Diagnosis 3: Condensation overflow at the drain pan 2:23 Diagnosis 4: Blown fuse at the HVAC control board 3:39 Confirm resolved problem at the thermostat