Set Appliance State:
Gas Off
Set Multimeter to AC Voltage 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Turn selector switch to DHW position
Turn selector switch to DHW position
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow fan to run.
Measure the voltage across the C - common (orange wire)
on the air pressure switch to a suitable earth point.
Voltage should be approximately 24V AC.
Measure the voltage across the C - common (orange wire)
on the air pressure switch to a suitable earth point.
Voltage should be approximately 24V AC.
Isolate as per TOPs P4
and refit as necessary