Theo says:
AThis sounds to us like a simple failure of the coolant temperature sensor (CTS) or its wiring, which is leading the ECU to believe that the engine is already warmed up.
The CTS looks similar to the sensor for the temperature gauge, except it has a snap-on plug with two connections inside. If the centre of the sensor is free to rotate in the metal housing, it’s likely that the internal wires have crossed, which would likely indicate a ‘hot’ engine to the ECU.
Remove the multiplug and use a test meter to measure the resistance between the pins of the CTS. If it’s zero (shorted) or infinity (broken connection) replace the sensor.