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E73 on a 6 year old nest

Community Member


  Just received an E73 error code on a nest thermostat that has been installed and working for the last six years.  Nothing has moved in the wiring or breaker box all other electrical items in the home working fine.

Not sure what to do? Is the nest past its design life?  


Platinum Product Expert
Platinum Product Expert

@Gorlt ,

An E73 error occurs when the power supply form the HVAC equipment has failed.  This is typically a problem inside the air handler of your HVAC equipment.  In the air handler is a step-down transformer that lowers the 120VAC or 240VAC voltage from the supply, down to 24VAC for the HVAC control system.  

The air handler typically has a 3 or 4 amp fuse on the output side of the step-down transformer.  If the air handler has a gas furnace, it will have a circuit board that controls the gas furnace and blower fan.  Go to the indoor air handler and look for power and thermostat cables entering the cabinet.  This will identify the location of the control components.  In this area you should see the Class 2 40VA transformer.  If this is a gas furnace, the transformer feeds power to the control board, and the control board feeds power to the thermostat.  You will need Volt Ohm Multimeter.  Set the meter to measure AlC Voltage.  Place on probe on R on the control board, the other on the C of the control board or the metal chassis.   You should read 24VAC +/- 5VAC. Less than 18VAC will not work or over 30VAC will damage the control system.  If you look at the control board, there should be an ATC blade type fuse.  If the fuse is blown, we need to find the short that blew the fuse.  I will wait to hear back from you as to you checking for voltage from transformer or if you find a blown fuse.  

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