Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Selector Switch in DHW Summer position
Hot Water Thermostat set to Maximum
Hot Water Thermostat set to Maximum
Gas On
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the DC Voltage across one of the DHW thermostat connections (Red Wire) at the thermostat and the B4 connection (Grey Wire) on the connector B Multi-plug on the PCB.
Repeat this measurement using the other DHW thermostat connection (Red Wire).
Note: The thermostat connections may be in different positions than shown. Due to the thermostat being modified.
Repeat this measurement using the other DHW thermostat connection (Red Wire).
Note: The thermostat connections may be in different positions than shown. Due to the thermostat being modified.
Isolate as per TOPs P4
and refit as necessary