Set Appliance State:
Gas Off
Set Multimeter to AC Voltage 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector switch in HOT WATER position
Gain access to PCB (use insulation mat as required)
Selector switch in HOT WATER position
Gain access to PCB (use insulation mat as required)
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Allow fan to run at high speed.
Measure voltage across connection 66 on connector R (blue wire, air pressure switch connection NO) and connection 52 on connector C (blue wire) on the PCB.
Measure voltage across connection 66 on connector R (blue wire, air pressure switch connection NO) and connection 52 on connector C (blue wire) on the PCB.
Isolate as per TOPs P4
and refit as necessary