Your boiler has detected that there is too much water inside your heating system, which is causing excess pressure.

DIY fix possible

Locate a radiator in your home, preferably one upstairs.Use a radiator bleed key to gently open the bleed valve.Release a small amount of water into a cloth or container until the pressure gauge on the boiler front drops to between 1 and 1.5 bar.Close the valve and check the boiler display to see if the error clears.