Your boiler's internal timer has lost track of its heating schedule, so it may not be switching on and off at the times you set, though it is often still able to heat your home.

DIY fix possible

Check the boiler display and reset the fault by pressing the reset or back button, as shown in your manual.Go to the time and date settings in the boiler controls and check they are correct. Set them if they are wrong, and check your heating and hot water schedules.Switch the boiler off at its on/off switch or isolator, wait about one minute, then switch it back on. If the code returns, or the time resets again, contact a Gas Safe registered engineer.