Yeah, it's definitely a valid test, the OP said that the fan is not coming on when the temps reach above the threshold. If there is faulty wiring between the sensor and fan, ie giving high or infinite resistance, then you will see this as the rad fan won't come on when sensor is unplugged, as it's still getting high resistance.
If the sensor is faulty, it will go to very high/infinite resistance, so no matter how hot the coolant gets, it will mover get low enough to switch the fan on. By pulling the plug, the circuit goes open circuit - zero resistance, & the fan comes on. That then shows the circuitry is working.
If you want to go in deeper (it means loosing some coolant), take the sensor out, and put an ohm meter across it. Put the end on a block of I've, and resistance should go up to around 1700ohms. Then put it in hot water, at 70degrees C it should be about 460ohms. 100 degrees should be around 200ohms. If the sensor is found to be working, then the coolant in the head is not reaching the sensor, meaning air lock. On older pumas, there are 2 sensors ( they are both there on newer pumas, but 1 isn't used). There's the 2 wire sensor for ecu, & 1 wire sensor for the gauge on the dash. If its a newer puma (digi dash) then the temp gauge on the clock will tell you if the sensor is faulty as it will go right up and stay there or more likely will stay right at the bottom and not move. But in older pumas, the dash is controlled by the 1 wire sensor, so the ECU and fan will see a different reading to the dash.
I hope that's a little clearer now, I don't think you were but picking mate, if I say something that don't make sense to you, it's best to query it!