[Edit: I just noticed Art posted while I was composing, so take note of his wisdom first, especially checking the main system ground point on the fuel rail.]

Did you also check port 6 in the diagnostic connector for ignition control module OBD codes? That could be very telling. It gets the same signal from the ECT and I would expect to see a 2-2-4 code. If that code isn't present then that says the problem is likely not wiring at the connector, but more subtle, such as a faulty ECT voltage that only upsets the ECU, not the ignition controller. In that case, check the sensor voltage as in Art's post.

OBD code 1-2-3 on port 2 says the ECU isn't getting a signal, leastwise not any kind of proper signal, from the ECT (coolant temp sensor, aka. block temp sensor). You more than likely still have a problem at the ECT that was replaced, either a wiring connection problem, the sensor not grounding to the block or the replacement sensor is faulty or incorrect. Check the wiring handywork there. Start by making sure the connector is properly seated.

Of note, that code can be raised if the ECT wiring is damaged and shorting to ground. Odds are that would be under the hood where work was being done on the connecter. You say the connector was replaced. Any chance it was wired backwards? The next likely place I can think of would be a damaged connector at the ECU with bent pins in the connector (pin 13) shorting to a neighbouring pin, such as from improper use of a meter probe into the connector (supposed to probe into the side slots).

You're likely aware there are two temp sensors under the intake manifold, so make sure you're checking the correct one. The forward one is for the dash temp gauge. The one at the back that's more difficult to reach is the coolant temp sensor for the ECU and ignition controller.

The usual symptoms for a bad ECT are idling rpm and running problems without an OBD code being set. So it's likely something fairly basic like wiring or the wrong sensor.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now