Set Appliance State:
Gas Off
On/Off Switch OFF
Set Multimeter to Resistance 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
DHW temperature control set to maximum
Remove the blue wire from the air pressure switch and make safe
Attach the +ve probe to the exposed terminal
DHW temperature control set to maximum
Remove the blue wire from the air pressure switch and make safe
Attach the +ve probe to the exposed terminal
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Appliance On/Off control set to On position.
Reset lockout as necessary.
Allow the fan to run.
Measure the resistance across the exposed terminal and the other terminal (black wire).
Reset lockout as necessary.
Allow the fan to run.
Measure the resistance across the exposed terminal and the other terminal (black wire).
Isolate as per TOPs P4
and refit as necessary