1994 850 Turbo 145k miles

The check engine would come on randomly and show code 1-2-3 (ECT sensor) for the last few months. It might be a few weeks before it came back or just a day or two, so I decided to swap out the thermostat and ECT sensor last weekend. Now about every time the car is driven the check engine light will come on after the car is running about 5 minutes (during warmup). Each time it is code 1-2-3, and will otherwise run and drive fine. If I shut off the car and clear the code, then restart (engine already warm) the check engine light will not reappear until the car is shutdown and cooled down for the night. The temp guage is exactly at the 3'o clock position when warm.

Could it be any leftover air in the cooling system (I drained and replaced it during install)? Any other ideas? I expected the new thermostat and ECT sensor would do the trick, and the car has been driven about 200 miles since replacement, so I figured any air would have bubbled out to the expansion tank by now.

Thanks,

Zack
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Zack Silver 83' 242Ti (230k)