My Dash Thermostat is reporting as high as the needle can possibly go, it gets there before the computer reads running temp (190-200).  My ODBII is reporting running temp after a proper warm up time.  The sensor is a 3 wire, combined sensor/switch.  Why would one read impossibly high, and the other work in parameter.  I've replaced the sender.  All this is after a rebuild so ground?  or other ideas please....

seems like a bad wire harness or ground.

 

there are 2 sensors built into that one 3-wire unit, one for the ECU, one for the gauge.

This is a common issue with the 3 wire temp sensors, especially aftermarket.

 

Make sure the wiring for the sender isn't cut/pinched.

If the wiring looks OK try a new OEM Subaru temp sensor.

One thing you could do to see if it is the sensor causing the trouble and not a short to ground issue with the wiring is remove the connection to the sensor. If the gauges goes low then you have proved the sensor is at fault. This is assuming that the gauge shows a hotter reading as the resistance of the sensor goes lower.