Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Expose PCB by removing plastic control box cover.
Mains Isolator Switch (ON/OFF Switch) in ON position
Selector Switch in DHW Summer position
Hot Water Temperature Control set to Maximum
Remove Air Pressure Switch connector (D19-D24) from PCB
Expose PCB by removing plastic control box cover.
Mains Isolator Switch (ON/OFF Switch) in ON position
Selector Switch in DHW Summer position
Hot Water Temperature Control set to Maximum
Remove Air Pressure Switch connector (D19-D24) from PCB
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
With fan running check short circuit resistance between terminals of air pressure switch, COM (Red/Black wire) and N.O. (White wire).
Mains power OFF and replace connector.
Mains power OFF and replace connector.
Isolate as per TOPs P4
and refit as necessary