Halstead Boiler Fault codes?

SERVICE MODE:
The appliance enters the SERVICE mode by turning the control
knob to ‘SERVICE’ (full clockwise). The LED displays a flashing
‘C’. In this mode the appliance runs at the minimum CH output.
This mode allows the gas valve offset and CO2 emissions to be
measured. To increase the output to maximum turn the control
knob back on and return to Service within two seconds.

A1
October 2013