NEST throws a 'Y' wire fault or E-103 error.

The NEST is detecting a problem with the wire connected to 'Y' on the thermostat. Typically the NEST on a call for cooling will not start the condensing unit and will short cycle the fan ( 5 to 10 seconds on and off ).

Possible causes in order of likelihood;

1) Condensing unit contactor bad or going bad.

2) Broken wire somewhere between connection to 'Y' on thermostat and contactor in condensing unit.

3) Problem with the common wire connected to 'C' on the thermostat.

4) Bad board in furnace ( only if wire lands on 'Y' in furnace and does something ).

5) Bad NEST thermostat.

If you need further help, please contact Johnstone Tech Support by phone or by writing up a new support request through https://techsupport.jswmi.com