Your boiler has detected that the water pressure is either too low or too high, or the gauge itself is reporting an incorrect reading.

DIY fix possible

Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.If the pressure is below 1 bar, use the internal filling link to top up the system.If the pressure is above 2.5 bar, bleed a radiator to release the excess pressure.