The electrodes on my sensor actually corroded all they way off when the car was only two years old (1990 GT). I replaced the entire sensor with a new Ford/OEM sensor and haven't had a problem since.
'clean...We're talking about the sensor that actually extends down into the coolant in the overflow tank. Take the entire cap off the OF tank and there will be a sensor on the end of a 12" extension. There should be two exposed metal tips. The coolant completes the circuit between the tips and keeps the idiot light off. Clean off the tips (if they're still there) to get good, uncorroded contact with the coolant. If the tips are gone, you'll need to replace the entire cap assembly.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team