let's suppose it's still good then check the wiring..

also disconnect it and start the engine to see if there's any change.. if
none then the sensor is bad or not reacting

on mine when I disconnect it, it idles bad, running rich till the coolant
temp is up...

the sensor could be still good but covered with a layer of deposit that
makes it less sensitive to heat and then it requires more heat to make it
work - mine was like that but I replace it anyway because it's the same
amount of work once removed.