F15 fault?

Hi, I have a F30E Theme Classic and have just replaced the 3-way valve as it had split (again). I have now installed the modified unit with the copper elbow on the front. After refilling and bleeding the boiler, I started it up again. The heating and hot water all worked fine for a day but now I am getting a 'F15' fault and flashing red light. I checked the manual which said that there is no gas but there is and I hadn't even touched anything to do with the gas... Can anyone help?