Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect modulating solenoid wires (blue connector) from the gas valve

Measurement:

Test 1:
Check resistance across connection 1 on ST16 and left wire
(when held as shown) on solenoid connector
Test 2:
Check resistance across connection 2 on ST16 and right wire
(when held as shown) on solenoid connector
Expect continuity on each test.
Test 3:
Inspect the general condition of the harness and plugs and
check the resistance of both wires to a suitable earth point.
Expect greater than 1M Ohm on each wire.