Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect modulating solenoid wires (blue connector)
from the gas valve
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.
Also inspect the general condition of the harness.
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.
Also inspect the general condition of the harness.
Isolate as per TOPs P4
and refit as necessary