The running radiator fan suggests the Engine Computer is what thinks the engine is overheating so it's turning the fan on. The instrument cluster is simply displaying the incorrect information it's being told by the Engine Computer.
The only thing you can try yourself is to unplug the coolant temperature sensor, then use a small piece of wire to jump the two terminals together in the connector to see if either condition causes a change in the gauge reading. If it does, suspect the sensor but that would be very uncommon.
In the past, testing this circuit was real straight-forward and easy. An unplugged sensor sent the gauge to full-cold, and jumping the two wires together sent it to full-hot. The problem is with the Engine Computer involved, either condition tells the computer it can't believe the sensor's readings so it assumes an overheating condition and turns the radiator fan on to protect the engine in case it really is overheating. It's a safety system built in to prevent damage from overheating in case it does occur and the computer is unaware of it.
Saturday, April 4th, 2015 AT 9:31 PM