<VV> NO Corvair - GM Radiator Question

[GYoungwolf at aol.com]

I changed the radiator in my daughter's '95 Lumina this past weekend (plastic 
tank had cracked at 200K), but now when the engine warms up, the Low Coolant 
warning light comes on. The coolant is fine and there is no air in the system. 
The sensor is about 2/3 of the way up on the right tank, but does not come in 
contact with the water. Does anyone know what might be wrong, or whether the 
computer needs to be reset, etc.?
Thanks,
Grant