Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove electrical connector to spark generator.

Measurement:

Measure for resistance across pins 1 & 2 of the spark generator connection (see note).
Expected resistance approximately 270 kOhms.
Note.
It may be easier to carry out this test with the spark generator removed from boiler.