Your boiler is signalling that the water pressure inside your heating system has risen too high, preventing it from operating safely.

DIY fix possible

Locate a radiator in your home, preferably one upstairs.Using a radiator bleed key, slightly open the bleed valve to let a small amount of water out into a cloth or container.Monitor the pressure gauge on your boiler's front panel until the needle drops back into the green 'normal' zone (usually between 1 and 1.5 bar).