Coolant Level Sensor?

Have an 03 Envoy. Check engine light is on. Check code. Says engine coolant level low. Check coolant. Full, right where it sould be. Check auto parts store, they have sensor. Go to Haynes manual, nothing about engine coolant sensor. Can anyone tell me where it is located?? It's pretty inexpensive, but I can't figure out where it is.
Thanks for any help.

Do you mean coolant temp is out of range or low? This is a different issue. What was the code?

Thanks for responding. Went back and had them check it again and read the read out myself. It is the coolant temp is lower than the thermastat.
So that would be the coolant TEMPERATURE sensor. Figure it must be the sensor as the temp gauge is in it's normal operating range.
Thanks again.

I'd get a scanner on this and see what the actual temp is, it could also be the the T'stat.