99 mustang gt coolant issues!

Look at where the upper radiator hose attaches to the intake manifold, is the crossover from the driver head to the passenger one, is it made of plastic or aluminum?

The plastic ones tend to crack, fix this with a revised intake manifold with an aluminum crossover.

Also, the sensor goes off if there isn't a good enough mixture of water and coolant, anything less that 75/25 coolant to water I think the sensor will go off. Also, if you go cleaning the engine, you have a chance of confusing the sensor. That has happened to me twice, and I knew what I did after I went to start the car.

If there is anything else you have recently done to the car it would be wise to tell us..

Good Luck!

1st, see if there are any codes
2nd, the sensor might be crudded up and not responding properly

If you take off the radiator cap you can see the float inside. Make sure that it moves up and down freely. Clean it if necessary. If that doesn't work maybe you need a new sensor.

I just had that same issue. My sensor had corroded on the end that went into the intake. I cleaned it and no issues thus far.