1997 Saturn SL1 what causing senor to fail

Hi I have a 1997 saturn sl1 and for about 2 years now I have been replacing the coolant senor about twice a year now. one in the summer and one in the very cold winter months. what is causing the senors to fail.l I know how to change them quickly and not lose very much coolant lol and the senors are only about $20 canadian. i bought an after market senor they do not work right the parameters are to far apart ( the fan does not turn on )if you get the senor from saturn the parameters are closer and the fan works the way it should.