2000 Other Saturn Models Speed sensor. Is that all that I

A while back, my "Check Engine" light began to turn on intermittently, causing my speedometer to cease working and cause my fan to run constantly with a high whirring noise. The funny thing is, I never have a problem with it when ithe weather is damp and rainy, only when it is dry and sunny. I had my car checked by a mechanic recently, and he scanned for a code, which came up "Speed sensor". He has ordered the replacement sensor and I will have that replaced when it arrives. I was just curious if anyone can advise why I have this problem based on weather changes, and if the sensor being replaced will eliminate the problem of the fan?