From a post on the C-320 list a few years ago on this very problem:
Well this has turned into an interesting project. For those late to the telling, my fuel guage is reading ‘Full’ all of the time, so I replaced my float unit in the tank and that didn’t cure the problem.
With no input from the list on my query as to whether there was any advice other than ‘replace the guage’, I turned to Catalina to order a new guage. They promptly referred me to Seaward where I wound up with Lonnie (562-699-7997 ext. 213). He was extremely helpful to me and when I expressed my intention of posting this report to this list, he volunteered that I should provide his contact information in the event anyone had further questions. They are very proud of their products at Seaward and clearly dedicated to customer service.