Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X5 from the Aquastat PCB

Measurement:

Measure the resistance across each of the wires in the harness from the
Aquastat PCB to the Ignition Control Box.
Pink on X5 to pin1 on control box connector.
Brown on X5 to pin 10 on control box connector.
Blue on X5 to pin 11 on control box connector.
Earth on X5 to pin 12 on control box connector.
Expect continuity / zero resistance on each of the wires in the harness.
Also visually inspect the harness for good condition.
Are the resistances measured as expected and is the harness in good condition?