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error Worcester
Error Code A21
No RF signal detected between the room thermostat and Programmer/Receiver.
? What Causes This
Poor RF communication or incorrect settings/positioning of units.
✓ How to Fix It
Check if units are in pairing mode, verify positioning for better signal strength. Adjust pairing settings via installer menu.
Source: page 19 of the official manual
From the manual
This error code appears in:
Worcester Greenstar Comfort II RF