Stuck thermostat or bad thermometer?

[jeremym]

Yesterday my temp gauge sat at the blue zone for an entire 50 mile trip. At the end of the exit ramp I turned the X off and back on and the temp gauge shot to the red and pinned. My heart sank. I then drove to the nearest parking lot and turned it off and back on. Back to the blue zone. Couldn't get the needle back to the red after many restarts - or the normal 12-o-clock position. I topped the coolant off with distilled water and still saw the same thing - blue zone. Drove to the destination, another 10 minutes on back roads and the gauge never moved.

Is this a stuck open thermostat or a fried thermometer? I'm doubting it's the gauge...but it could be.

I can provide more details if needed. Thanks.