Excess heating

Sounds like a plan. Let us know what happens.

I would pop the temperature sensor out and stick a gauge in there and see what the temperature actually is. 143 wouldn't be hot enough to cause problems. Testing it may show if it's the gauge or sender. It sounds like the wrong sender.

Normally the fans won't turn on until about 210-218 degrees. With everything else it sounds like the ECM isn't responding to the coolant temperature sensor to turn on the fans. That points to the ECM as the failure point as the rest of the system is working.

The only item really left that could cause it. Normally on those if the wiring was broken it would tun the fans on and if the sensor is unplugged it would do the same. In this case it sounds like an internal ECM fault, either a broken trace or some 5 cent part failed on the board.

Unless the gauge is wrong. I would put a mechanical temperature gauge on it before swapping the ECM out just to see if the problem is the gauge or in the dash. If the mechanical says the temp is at 210 or so and the fans come on then it isn't the ECM but a bad gauge or gauge driver.

Yes it's behind the glove box area. See the images below. You or the mechanic will need to swap the PROM and if installed the Cal-Pak chips over to the replacement unit.

This is from the service info on that car -
Some control module's may have problems due to cracked solder joints on the circuit board. These internal control module problems can cause the following symptoms

Failure to start or vehicle is stalling.
The "CHECK ENGINE" "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR" light will flash or light up, but no trouble codes will be present.
Vehicle instrument panel displays may be inoperative.
The control module may or may not communicate with the scanner.
Other intermittent driveability problems.

Incorrect PROM or MEMCAL removal and replacement can create solder joint problems or aggravate an existing condition. See PROM or MEMCAL INSTALLATION for proper procedures.

If a solder joint problem results in a "hard" failure, normal test procedures will usually pinpoint a faulty control module. Many symptoms caused by poor solder joints in the control module result in intermittent problems, but they may be hard to duplicate during troubleshooting. Control module's with solder joint problems are sensitive to heat and vibration.

When replacing a production ECM with a service ECM, transfer the broadcast code and production number from the production unit to the service unit. Also, during replacement, the PROM must be removed from the ECM being replaced and transferred to the new unit. Some ECM's are equipped with another chip called a Cal-Pak. If the ECM if equipped with a Cal-Pak chip, it will be located next to the PROM and must be transferred along with the PROM.

1. Disconnect battery ground cable.
2. Then disconnect two computer electrical connectors.
3. Remove computer mounting hardware.
4. Remove computer from passenger compartment.
5. Reverse procedure to install.

You are welcome.