If you have high fan when you unplug the blu sensor, but it does not come on when the coolant temp gets to 105/107, then the sensors values are off spec. A new sensor will work..
There is also a modification we do by putting a resistor across the sensor that allows the cut-in spec of the sensor to drop to around 100C..you may want to do that.
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There should be a distinct difference from low/high speeds, so if you do not see that, then the fan motor is suspect, along with the other bad one.
Another trick is to put a cabin on/off toggle switch across the wires I had you jumper to test for low fan..that allows you manual control of the fans from the cabin..cheap mod and comes in handy when you run into overheat condition like you did the last time.
Both fans must run, so start there.........sometimes the brushes get crap in them and a sharp blow at the blade fron hub w/rubber hammer will get one going again

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A Dalton