Set Appliance State:
Set Boiler Control Thermostat to MAX position
Set Multimeter to AC Volts 
CH Demand
Isolate Mains Power to appliance as per TOPs P4
External controls calling for heat
Gain access to PCB
Use insulation mat as necessary
External controls calling for heat
Gain access to PCB
Use insulation mat as necessary
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the voltage across the normally closed terminal (yellow wire) on the air pressure switch and the neutral (marked N) on the main connector block.
Expected voltage 230 volts AC (mains voltage)
Expected voltage 230 volts AC (mains voltage)
Isolate as per TOPs P4
and refit as necessary