Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
NOT CURRENTLY USED
NOT CURRENTLY USED
NOT CURRENTLY USED
NOT CURRENTLY USED
CH temperature control knob to max
Unclip and remove the plastic covers from the wires for the flue thermostat
Attach crocodile clips to the wires
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset appliance to ensure it is not in lockout.
Allow boiler to complete its self check.
Allow the boiler to fire for heating.
0V DC should be displayed on the DVM.
Observe the voltage when the appliance shuts down prematurely.
Does the voltage remain at 0V DC or does it increase to approx 23V DC? (Voltage may read as negative.)
Note:
Voltage may only change briefly before returning to 0V DC.
Voltage remains at 0V DC
Voltage increases to 23V DC