Set Appliance State:
Mains Power Off
Set Multimeter to AC Volts 
CH Demand
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
If boiler lights allow it to operate until burner shuts off early.
Measure the voltage between the yellow wire marked "NO" at the plug for the air pressure switch (RHS, refer to pictures) to a suitable earth point.
Is mains voltage still present?
Measure the voltage between the yellow wire marked "NO" at the plug for the air pressure switch (RHS, refer to pictures) to a suitable earth point.
Is mains voltage still present?
Isolate as per TOPs P4
and refit as necessary