Set Appliance State:
Mains Power Off
Set Multimeter to AC Volts 
Turn Hot Water Tap On
Mains Isolator Switch (ON/OFF Switch) in ON position
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Test 1:
Measure voltage across output terminals of overheat thermostat connections marked 12 (Brown wire) and connection marked 22 (Blue wire).
Test 2:
Measure voltage across input terminals of overheat thermostat connections marked 11 (Brown wire) and connection marked 21 (Blue wire).
Measure voltage across output terminals of overheat thermostat connections marked 12 (Brown wire) and connection marked 22 (Blue wire).
Test 2:
Measure voltage across input terminals of overheat thermostat connections marked 11 (Brown wire) and connection marked 21 (Blue wire).
Isolate as per TOPs P4
and refit as necessary