Set Appliance State:
Mains Power Off
On/Off Switch OFF
Set Multimeter to AC Volts 
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Appliance On/Off switch On.
External programmer calling for DHW.
Measure voltage across terminal marked HWR and terminal marked N on the main 12 terminal connector block below the control panel.
External programmer calling for DHW.
Measure voltage across terminal marked HWR and terminal marked N on the main 12 terminal connector block below the control panel.
Isolate as per TOPs P4
and refit as necessary