Your boiler is struggling to breathe because something is blocking the passage of air or exhaust gases through the flue pipe.

DIY fix possible

Check the external flue pipe terminal outside your home for obvious obstructions like bird nests, leaves, or debris.During winter, check if the flue outlet is blocked by heavy snow or icicles.Reset the boiler by turning the selector switch to 'R' for five seconds after removing any external blockage.