1. The sensor itself could be bad. If you have a multimeter you can check the probe for it's resistance. Normally it is just 'open' but if it's not around 1k to 6k ohms at room temperature then it's probably a bad sensor.
2. Firmware. If you can update that (d/l available here on EP-UK) to the latest one, it may fix it if it's been corrupted a little.
Worth trying both, and if you aren't comfortable with doing the first, then the second is a good start too