Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Disconnect one of the Red spaded wires from the Overheat Thermostat and ensure that the disconnected wire is safe and cannot touch any part of the Boiler.
On/Off Switch On.
Disconnect one of the Red spaded wires from the Overheat Thermostat and ensure that the disconnected wire is safe and cannot touch any part of the Boiler.
On/Off Switch On.
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the DC Voltage across connection 1
(Red/Black wire, LED positive) and connection 4
(Red/Blue wire, LED negative) on the PCB connector multiplug.
Reconnect Lead.
(Red/Black wire, LED positive) and connection 4
(Red/Blue wire, LED negative) on the PCB connector multiplug.
Reconnect Lead.
Isolate as per TOPs P4
and refit as necessary