Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Water heater switch set to constant
Remove the upper electronic flame control device
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across connection 4 (blue lead) and connection 8 (brown lead) on the control module.
Refit the electronic flame control device after test.