Set Appliance State:

Mains Power Off
CH Demand
Programmer or selector switch set to heating and hot water
Heating temperature selector to maximum
Hot water temperature selector to OFF
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to fire.
Does the temperature of the CH flow pipe increase signifying the diverter valve has opened?