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
Gain access to PCB
Use insulation mat where necessary
Remove the black lead from the air pressure switch and make safe
External controls calling for heat
Gain access to PCB
Use insulation mat where necessary
Remove the black lead from the air pressure switch and make safe
External controls calling for heat
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow fan to run.
Measure the voltage across the exposed terminal (NO) on the air pressure switch and the neutral (marked N) on the main connector block.
Expected voltage 230 volts AC.
Measure the voltage across the exposed terminal (NO) on the air pressure switch and the neutral (marked N) on the main connector block.
Expected voltage 230 volts AC.
Isolate as per TOPs P4
and refit as necessary