Your boiler has detected that the gas flame is not turning off when it should, which is preventing the system from running safely.

Error with gas valve – switch off delay

What causes this fault?

This fault occurs when the boiler's control system detects that the gas flame is still burning even after it has told the gas valve to close. The most common reason is a mechanical failure within the gas valve itself, but it can also be caused by a faulty circuit board incorrectly sensing a flame that isn't actually there.