The boiler repair company expects the fine from the Financial Conduct Authority (FCA) on top of the £24.2 million already set aside to cover the compensation process for customers and overhauling working practices.
The investigation, 700 UK job cuts and falling UK sales sent pre-tax profits plunging by more than half to £66.5 million for the year to the end of March.
Walsall-based Homeserve has been under a cloud since first revealing problems with its sales calls in October 2011. It said the FCA's investigation will take a "number of months" to complete and that £6 million was its best estimate on a potential fine and associated costs.
The probe is thought to centre on mis-selling of policies and poor handling of customer complaints. The company said it spent £9.5 million of its £24.2 million provision over the year on contacting and compensating customers, plus reviewing complaints from winter 2010.
Homeserve's policies include boiler and central heating breakdown cover from £4 per month and insurance against blocked drains from £2.50 per month. It has tie-ups with major utility firms such as Thames Water and United Utilities.
The company was founded by chief executive Richard Harpin, who has about 12% of its shares.
Mr Harpin said: "Our products and services continue to meet clear customer needs and we are confident that our business model can continue to deliver long-term value for all stakeholders."
Shares in the company surged more than 11% on relief that the estimated fine was not greater. Homeserve held its total dividend at 11.3p per share.