Your boiler has stopped working because the waste water pipe is blocked, likely by debris or ice if it is cold outside.

Appears if the condense trap/pipe has become blocked.

What causes this fault?

This fault occurs when the boiler detects that waste water, known as condensate, cannot flow away through the drainage pipe. The most common cause is a physical blockage in the internal trap or the external pipework, which triggers a safety sensor to prevent water from backing up into the boiler's combustion chamber. If the water cannot escape, the boiler will shut down immediately to protect its internal electrical components from water damage.