Set Appliance State:

Mains Power Off
Mains Reset Switch OFF
Set DHW temperature selector to ECO
Set CH temperature selector to OFF
Ensure the boiler is cool so that there is demand for preheat
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Mains reset switch On.
Allow boiler to ignite and run preheat mode.
Note:
Boiler should fire and heat primary circuit to correct temperature and shut down in approximately two minutes. This time will vary due to starting temp of boiler.
Does preheat complete?