Morning,
Mate of mine (rebuilding a Series11 Landy and will be registering on here soon) has an Audi with a small electrical fault.
The rad fan is running continually, soon as ignition is switched on the fan runs and continues when car is switched off (not sure how long for 10-15mins prob).

Any Audi experts about ??

On most modern cars if the engine management runs a test program at start up if detects an out of range reading from one of the temperature sensors it will turn the fan on.

It could also be caused by the AC

Better to buy a diagnostic scanner or dongle+software that has VAG special functions to do a proper check rather than change random components blindly.

temp sensor or rad switch in rad

^^^^^^^^
Wot he said!

Once had a similar problem with a Passat that was caused by water getting into the fusebox causing corrosion to build up on the printed circuits and short out whennthe fan operated. Bodged it up as a temporary measure by wiring the fan up independently of the fuse box .

My son had this problem on his Golf, fixed it with a new coolant temp sensor.