Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Connect a performance tester to the gas valve pressure out
(uppermost of the two test points)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the fan pressure during the fan and pump purge period.
Fan pressure will read as negative
and should be 1.4mb or greater (e.g. 2mb).
Note:
When performance tester is removed test point should be tightened and checked for gas tightness.