IIRC its a maintenance code.
Try turning the power supply off to the boiler for a while. It may clear when you turn it back on again.
Failing that hopefully someone else can help.
EDIT:- An excerpt from the MI's
8.24.2.4 Using a screwdriver lightly press the black
toggle behind the combustion switch cap once
only. The appliance will light at maximum rate.
At this point the digital display will show CO to
confirm the appliance is in combustion test
mode.
8.24.2.5 Analyse flue gasses on analyser.
8.24.2.6 Once the analysis is complete lightly press the
black toggle once to disengage the combustion
mode. The display will revert to showing
appliance temperature on the digital display.
Looks like someone has been playing about with it