What is causing my radiator fan to constantly run on my 2000 Mercury Villager?

I bought this van used. The radiator fan constantly runs as soon as the engine is started. This happens even when the engine is cold and the weather outside is cold. The fan then turns off when the engine is shut off. What could be the cause and how do I troubleshoot this? Things I have already done: 1.... show more I bought this van used. The radiator fan constantly runs as soon as the engine is started. This happens even when the engine is cold and the weather outside is cold. The fan then turns off when the engine is shut off.

What could be the cause and how do I troubleshoot this?

Things I have already done:
1. Thermostat has been replaced.
2. Engine Coolant Temperature Sensor has been replaced.
3. Three fan related relays have been tested and presumed to be in good working order.

After researching online, one person has found that replacing the ECM under the glovebox has fixed the problem.

Does anyone have any further DIY suggestions? Thank you to anyone who can provide insight!
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