Set Appliance State:

Mains Power Off
CH Demand
Ensure no DHW demand
Selector switch in CH & HW mode
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Is the diverter valve motor in the CH position (bottom)?
Note:
It may take up to 15 seconds after burner ignites for motor to move into CH position.