Volvo Hour Meter stopped working

Hi There I'm new member with a "MOBO" actually an Antares Series 9 with a single Volvo KAMd43.
Really pleased with boat, no real problems over last couple of years sailing on Loch Ness and the West coast of Scotland even in heavy seas.
I'm an ex Marine Engineer hence the interest in Motor boats.
Only problem encountered has been "Engine hour meter" has stopped functioning after approx 400 hours. I cannot find anything wrong with wiring or connections to unit.
Tacho is still functioning OK.!!
Is this typical of the volvo unit failing and can hour unit be repaired or replaced without replacing whole tacho.??
Anybody got any ideas. ??
Cruise Scotland enjoy life