The sensor is a resistive load sensor. The resistance changes as the motor gets hot. Solder the wires back onto the board and whilst keeping the model off of the ground, put it into drive mode. If it doesn't go, then wet your finger and bridge the two solder points on the sensor. Don't worry, you will not get any electric shock as the voltage and current are too low.
If it still doesn't drive, then wiggle the motor wiring at the motor. If still nothing, then press lightly on that sensor pcb. There may be a broken track on it. If it is broken, then you cannot solder it. You then need to go to Maplins (if in the UK) and buy a small bottle of pure silver paint. Where the solder points meet the circuit resistive track (it'll be black carbon looking), paint one blob of silver conductive paint onto each of the track beginnings. Let those dry (five minutes) and then try it. It should work. The silver paint will only work if the wet finger trick on the black carbon track works. If no carbon track, then it may be a thermal resistor (thermistor). Replace it. Maplin sells them too, so just buy a few different types (very cheap) and temporarily solder one in place until it works.
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