The fog horn on the Hanois Lighthouse off Guernsey has been silenced after sounding continuously for two weeks, despite there being little or no fog.
Peter Gill, Guernsey's harbour master, said the fog horn was turned off after the problem came to light on Tuesday.
Trinity House, which looks after a lot of Britain's lighthouses, said an automatic sensor had failed and they were now operating the signal manually.
Engineers are expected to travel to the lighthouse on Monday to make repairs.
Captain Gill said: "The automatic sensor on the top of the lighthouse detects any reduction in visibility, whether that be from rain or fog, or dust or even snow and automatically switches the fog horn on and then off again when the visibility improves."
He said it was when only turned off when "BBC Guernsey phoned the harbour office and asked why it is still running and of course we can't hear it at the harbour office".
He said: "We phoned Trinity House and they said, 'well we did switch it on when there was fog a couple of weeks ago and it's still switched on', so they switched it off."
The Hanois Lighthouse was automated during 1995 and the last keeper left at the start of 1996.
It was also converted to solar power with panels mounted around the lower part of the helideck support structure.
It takes its name from the group of rocks, Les Hanois, on which it sits south west of Guernsey, marking the western most point of the Channel Islands.