this problem happen to me. i replaced the coolant temp sensor and the temp started to get high and fans wouldnt kick on til high temps. i would just change the old one back if its still working thats what i did. if not try to find a better replacement

 

The coolant temp sensor (which controls the fans), and the temp sending unit for the gauge (what the OP would have needed to change to affect the gauge) are two different sensors.

 

Thanks again for your replies.