Set Appliance State:
Mains Power Off
Mode selector switch to Off/Reset position
Set Multimeter to Resistance 
Turn Hot Water Tap ON
Hot water temperature control set to maximum
Disconnect the red wire (marked 1) from the air pressure switch and make safe
Disconnect the red wire (marked 1) from the air pressure switch and make safe
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Mode selector switch to DHW position.
Measure the resistance across the exposed terminal (marked 1) and the terminal marked 3 (black wire) of the air pressure switch.
Mains power off.
Reconnect wire after test.
Measure the resistance across the exposed terminal (marked 1) and the terminal marked 3 (black wire) of the air pressure switch.
Mains power off.
Reconnect wire after test.
Isolate as per TOPs P4
and refit as necessary