Check that the wiring for the water temp sensor (2 pin connector at the back of the head on the passenger side) is OK. Unplug it and pull back the rubber boot and give it a good looking over.

If the wiring looks OK, remove the ECU hairbrush and measure the resistance between pins 11 and 29. Should be around 4500 ohms (engine cold) to 950 ohms (engine warm). If resistance high, change sensor or check wiring again.