Your boiler has detected that the water pressure in your heating system is dropping too quickly, which usually points to a leak or trapped air.

Pressure drop in heating system too rapid. Too many unexpected pressure drops within a short time period. 1. Cause code can appear if system is pressurised and the intelligent filling loop activated and then the system vented. 2. Switch off the intelligent fill in the system menu, re-pressurise manually and vent the system. 3. Ensure the correct pressure. If cause code does not clear power off appliance.