Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the connector from the flame sense electrode
Remove rectification lead from right hand side of main PCB

Measurement:

Measure the resistance of the rectification lead.
Expect continuity / zero resistance.
Also visually inspect the condition of the harness
and its connectors.
Note:
1. Rectification lead does not have to be removed
from the boiler for test but is shown removed for clarity.
2. The rectification lead may differ in appearance from the one illustrated but the resistance test is carried out in the same way.