E74 Wiring Error Message

[TMM1]

Why do I get this error message?  The system has been working fine for the last month.  Now, this message appears.

[David_K]

Google has good guidance and troubleshooting steps for this error:
Troubleshoot thermostat power issues during hot weather (E3, E4, E23, E24, E73, E74, or E195 help co...