Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the red leads from the CH differential pressure switch and make safe
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Test 1:
Measure the resistance across the microswitch.
Expect open circuit.
Test 2:
CH demand, allow pump to run.
Measure the resistance across the microswitch.
Expect continuity / zero resistance.
Measure the resistance across the microswitch.
Expect open circuit.
Test 2:
CH demand, allow pump to run.
Measure the resistance across the microswitch.
Expect continuity / zero resistance.
Isolate as per TOPs P4
and refit as necessary