Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the two wires from the outdoor sensor connections (terminals 3 and 4 at the boilers main terminal strip) and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

There is no need to wait for the fan purge to finish before commencing this test.
Has the fault code F39 for the outdoor sensor cleared?