Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
CH Demand
Selector Switch in CH/DHW Winter position
Heating Thermostat set to Maximum
Heating Thermostat set to Maximum
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the DC Voltage across one of the Thermostat connections marked A1 or P1 (Violet wires) and the B4 connection (Grey wire) on the Connector B multi-plug on the PCB.
Repeat this measurement using the other Thermostat connection marked A1 or P1 (Violet wire).
Repeat this measurement using the other Thermostat connection marked A1 or P1 (Violet wire).
Isolate as per TOPs P4
and refit as necessary