Your boiler has noticed that the water pressure does not jump up as expected when the internal pump starts, usually due to low water pressure, trapped air, or a faulty sensor.

No sudden change in pressure was detected on turning on the pump - Water pressure sensor and/or pump faulty (pump seized - check system water for contamination) Air in heating system, automatic air vent system faulty Too little water in the unit; check adjustable bypass; Connect expansion vessel in return;