Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to the boilers main terminal strip
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between the leftmost connection marked "FROST STAT" to the "MAINS IN" N connection at the mains terminal strip.
Note:
If no frost stat is fitted select that option below.