Dometic air conditioning roof unit (CA1000)

Can anyone give me some guidance on fault where the extracting condensation pump keps running and shows error message FO4.

The trouble shooting guide gives some help on the cause but the remedies are to contact an authorised workshop.

Will give them a call next week and also ask Autosleepers for some guidance, but like to have a quick look myself first.

Regards to all

Terry