Was just wondering do you run with the aircon on all the time as this would also cause the fans to run?
And of course fans continuing to run when key removed is usually when a DPF regen has been interrupted by switching of the engine.
It depends how long you want to carry on playing parts darts,
as said earlier it needs code reading on a Vx compatible reader,an obd reader will miss so many manufacturer specific codes,also useful for catching a data recording at the point the issue occurs.
Being able to pinpoint a fault like this becomes harder as vehicles electrics get more complicated. More expensive diagnostic products are now an essential item if you want to have any chance of finding the fault quickly.
It could be an easy fix like an open circuit radiator coolant sensor or associated wiring, but of course may not be guessing can be expensive.