critical Honeywell

Error Code AC LED Off, DC LED Off

Loss of AC power and no DC output — the standby battery is discharged or not present, and no power is available to the system.

? What Causes This

AC power failure combined with battery being discharged, missing, or dead.

Check AC power availability and reconnect if needed.
Verify the battery is properly connected (red positive terminal to battery positive terminal).
Allow battery to charge for several hours if it was discharged.
If problem persists, replace the battery (BAT-3, 4.0 amp hour).

Source: page 2 of the official manual

From the manual

This error code appears in: