Volvo: Water coolant sensor Volvo S70, water coolant, temp gauge

Question
Hi Ray,
      My car (1998)Volvo S70, 2.0 10V, decided to boil (steam + water from expanstion tank)while in a traffic jam but the temp gauge inside the car read NORM (3 o'clock).I had NO other warning on the dashboard at all. We managed to get some 2nd hand sensors x2. We then managed to obtain the technical information about the sensors which meant we knew what the resistance should have been at what temp. This showed that the sensor was ok, BUT when plugged in to the car and a flame to the sensor tip with the engine running the cooling fan started. The temp gauge DID also go up and above the norm which is what we expected. Using another sensor with no flame put on it but measured in a kettle of water previously, We put this sensor back in to the car and in to the thermostat housing and ran the car up to normal operating temp, The fan did NOT cut in. But we cleaned the electrical contacts where the sensor plugs in to the car with contact cleaner and fan now cuts in, My question is this, The fan now cuts in while to gauge reads 3 o'clock. I think it cuts in a speed one, will the temp gauge riase if the engine does genuinly get too hot? Also, the pipe from the thermostat to the radiator gets very hot and very stiff (you can't squish it) is this ok??
Thanks alot
John

Answer
John,
So you have a good sensor and the fan is able to work, temp seem ok is the t/stat good? I would repalce it and see if that will help. So many t/stats have gone bad good practice to replace one with some age. The hose you described sounds like it is old and dried up, I would also replace that and any other hoses that appear old and dried.
If the sensor you have is good the gauge should go up if the cooling system gets too hot.