I'm guessing you used an aftermarket ECTS and not a Genuine GM part.
If you had the ORIGINAL GM part still in there from the factory, it had failed. When it does, it often leaks coolant onto the contacts of the ECTS connector. This leads to corrosion. This particular circuit is extremely sensitive to corrosion, as the voltage drop across the ECTS is being measured, and as the temp goes up, the resistance of the ECTS goes down.
The corrosion adds a bunch of resistance to the circuit. This results in a bogus determination of coolant temp by the PCM, which thinks the coolant is cold and therefore never turns on the fan.

Also, when the ECTS fails, ut usually fails open circuit, which corresponds to an infinite resistance which leads the PCM to think the coolant temp is always -40 -- and therefore the fan is never told to turn on.

1)Check the contacts on the ECTS connector for corrosion. If corroded (usually green), replace (see below)
2)Check the circuit wiring where it meets the contacts of the connector. Broken or damaged wires here will leave the circuit with too much resistance or open circuit (see above).

Your ride is 20 yrs old. Treat it to a new Genuine GM ECTS (all brass design) AND ECTS connector with a wiring pigtail.

Amazon Cost:

Genuine GM 21025106 Engine Coolant Temperature Sensor $12.50

GM 12117087 Engine Coolant Temperature Sensor Connector Amazon: $20

Solder and shrink wrap, do not butt splice.

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The AutoZone /Advanced/etc sensor is not application specific, and, per one of our members who has worked for both, never buy electrical parts form either store. Their return rate is over 50%. Cheaply made, some DOA.

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