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error Honeywell
Error Code E99
General error. A miscellaneous error has occurred during operation.
? What Causes This
Unknown or miscellaneous system error.
✓ How to Fix It
Remove thermostat from wallplate for 10 seconds, then reconnect it (see page 26).
Source: page 62 of the official manual
From the manual
This error code appears in:
Honeywell RTH6580WF Wi-Fi Smart Thermostat