Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the fan to run, burner to ignite and establish
and the appliance to run with a DHW demand.
Measure for AC voltage across pins 69 & 70 on connector CN12 whilst the burner is lit.
Expect mains voltage.
Turn hot water tap OFF.
As soon as the demand ends the voltage should return
to approximately 0 Vac.
Does the voltage behave as described above?