Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to AC Volts 
CH Demand
Air heater switch set to constant
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Test 1.
Measure voltage across connection 13 (black lead) and connection 4 (neutral, blue lead) on the control module.
Test 2.
Measure voltage across connection 14 (white lead) and connection 4 (neutral, blue lead) on the control module.
Mains voltage should be present on both tests.
Measure voltage across connection 13 (black lead) and connection 4 (neutral, blue lead) on the control module.
Test 2.
Measure voltage across connection 14 (white lead) and connection 4 (neutral, blue lead) on the control module.
Mains voltage should be present on both tests.
Isolate as per TOPs P4
and refit as necessary