Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
CH Demand
Remove Yellow wire (COM) and Grey wire (NO) from water pressure switch, making a note of their location
System Pressurised
System Pressurised
Gas On; Pilot On
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure short circuit resistance across Water Pressure Switch COM (Yellow) and NO (Grey) connections.
Isolate as per TOPs P4
and refit as necessary