Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect plug X8 from PCB
Ensure link on "Opentherm" terminals is still fitted
Ensure link on "Opentherm" terminals is still fitted
Measurement:
Measure resistance across pin 2 (black / yellow) and pin 3 (violet) on connector X8.
Resistance should be short circuit (zero resistance).
Resistance should be short circuit (zero resistance).
Isolate as per TOPs P4
and refit as necessary