Your boiler has detected that the water pressure inside the system has dropped too low to operate safely.

DIY fix possible

Locate the external filling loop, usually two silver valves connected by a braided metal hose underneath the boiler.Open both valves slowly to let water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close both valves firmly and check for any leaks; the fault code should clear automatically.