Engine coolant over temperature when ignition on

Hello,

A few days ago, I received an Engine Coolant Over temperature warning on my car (turbo) out of the blue after a twenty minute drive to work in the morning.

That day I checked the expansion tank level, and it was low, so I opened the cap and added about 600 ml of distilled water, and closed the cap.

No trouble for a day, then started intermittently fluctuating temperature randomly while driving, and throwing the over temperature warning.

I limped by for three days with an occasional warning, where I would pull over, and open the expansion tank cap for a couple of minutes. It would bubble furiously for about forty five seconds, then stop. I had recap it and drive on.

Third day, as pulling into driveway gave engine coolant over temperature and went into low power mode.

Now It will immediately peg the temperature gauge to high (right) and give engine coolant over temperature whenever the ignition is in the "on" position.

I checked the coolant temperature sensor, and there appeared to be coolant leaking by it. I cleaned up the coolant, and removed and inspected the sensor.

The o-ring was degraded and there was some mild patina on the sensor where it contacted coolant.

I cleaned the sensor with scotch brite, cleaned with alcohol and dried thoroughly, and replaced the o-ring. I also picked up a replacement sensor from AutoZone.

With the existing cleaned, or the new temperature sensor, it will still immediately peg the temperature gauge to high (right) and give engine coolant over temperature whenever the ignition is in the "on" position.

If started, the vehicle will only run at idle.

If I disconnect the sensor, and turn the ignition to "on" I get the same behavior, except the temperature gauge will peg high, then peg low, then return to high.

I do not have an ODBII handy, is there some way to re-set the system, or am I looking at possibly some sort of ECM/electrical issue?

Any help is appreciated!