Engine coolant sensor testing

i've noticed on a couple of occasions recently that my 2004 TF engine fan has started coming on when i wouldn't expect it ( this morning after the 13 mile drive to work in around 16°C), so i'll check the contacts on the temperature sensor first. has anyone tested these sensors in boiling water, and if so what temperature do they open / close at?

It would be easier to check live data in the OBD2 info.

The sensors are linier rather than analogue on or off.

Tried checking last night, but my cheap OBD2 reader doesn't give all the outputs.

How often / when should the fans run during normal use at this time of year? this week after a half hour run home both my radiator and engine fans are running, but engine temperature on the dash gauge is steady just below mid-way ( where it normally sits). Radiator fan stops first.

Gavin is your "normal run" home in at slow speeds? Its all related to the flow of air through the rad and side intakes, the faster the car travels the greater the volume of air keeping the coolant cooler. My fans rarely kick in on the motorway driving at 50+ mph average speed, but after a 40 mile drive to work when parking up the intake fan may start up for a few minutes.
This is assuming there are no blockages or restrictions in the cooling system, and the engine/fan temperature sensor is ok.

My 13 mile drive is mainly on open roads. This week on two similar days, on the first both fans were running when I got home, on the next neither, so I suspect the sensor is playing up.

The "normal" position on the dash gauge covers 88' all the way up to 110', these are the normal operating temps of the K series.

To check the exact coolant temp you need to read the OBD live data.
The ECU controls the front radiator cooling fan/s.

You can find information on sensor here, which might help in testing the actual sensor:

http://www.tom-seven.stillgame.ch/wiki/doku.php?id=vvc_control&do=index