The boiler has noticed the water pressure in your heating system has moved outside its normal range, which is usually a sign that water needs topping up or there is a small leak somewhere.

DIY fix possible

Make sure the boiler is switched off and has cooled down, then look at the pressure gauge on the front of the boiler. A normal reading is roughly 1.0 to 1.5 bar when the system is cold.If the reading is below about 1.0 bar, use the filling loop (usually a flexible silver hose under the boiler with two small levers or taps) to slowly add water until the gauge reads around 1.5 bar. Close both ends of the filling loop tightly afterwards.Switch the boiler back on and check the gauge again after a day or two. If the pressure keeps dropping, or the warning returns, check for damp patches or drips around the boiler and pipes and contact a Gas Safe registered engineer.