When the temp outside is below 40 deg or so my ABS light will stay off.  I drive 50 miles to work daily. If it gets above 50 degrees or so then the ABS and Brake light will come on. 

 

Anyone have any ideas as to why this might act this way?

 

I have replaced the rear wheel sensor but not the front wheel sensors.

 

Thanks in advance.

That is a strange one! My first suggestion is to see if you could get someone to scan the ABS for codes.