You might want to remove the connector for the sensor, and clean it and contacts on the sensor with small wire brush, spray contact cleaner. If there was conductive corrosion between the two contact points (green gunk), it would be a resistor in parallel with the sensor, and lower the effective resistance the ECU sees. And to the ECU, that means increased heat. With mine disconnected (different system) I get a -40F reading for coolant temp. On a D2, removing the wire should make code P0117 show up. Might try a good cleaning to remove any electrical stray resistance issue, re-check readings with Ultra Gauge.