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By connecting a temporary wire all you did was short the circuit (just hope you didn't cause any damage). Renault electrics just don't like being shorted or sudden voltage changes. Ideally it would be better to test the resistance of the coolant sensors. The resistance at cold should be around 3500 ohms and about 300 when warm. The glow-plug timer unit also needs to sense the coolant temp via one of the sensors - so if it gets a faulty signal it will not be able to adjust itself corectly.
If the temp sensors and their connections are OK it could possibly be either a duff timer unit or the glow-plug relay
Currently in madnoel10's garage:
Honda Civic 1.4l