Your boiler is currently set to a manual testing mode used by engineers to check emissions and is not experiencing an actual part failure.

DIY fix possible

Locate the 'Chimney Sweep' button (symbol looks like a small brush or a circle within a triangle) on the control panel.Press and hold the button for at least 5 seconds until the display returns to the normal temperature reading.Switch the boiler off and back on at the fused spur switch to reset the operating mode.