Could be a duff sensor, or the wiring to the sensor.

Coolant Temp Sensor Checking - info courtesy of Simon @ SECS
1) Ensure ignition is OFF.
2) Remove rear of ecu plug cover and insert plug back into ecu.
3) Turn ignition back on (do not start engine)
4) Check resistance between pins 29 and 11. Should be 800 to 5000 ohms (depends on temp). If outside this range, check sensor and/or wiring.