MacBook Pro fan always on
If it is the same as mine, it was fixed by:


Opening Activity Monitor and see if any background processes are running.
If there are, then quit them.
If that didn't work, you might need to do a SMC. System Management Controller reset. 


To reset the SMC:

Shut down the computer.

Plug in the MagSafe or USB-C power adapter to a power source and to your computer.

On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.

Release all the keys and the power button at the same time.

Press the power button to turn on the computer.

On MagSafe power adapters, the LED might change states or temporarily turn off when you reset the SMC.

To read more about troubleshooting the fan read this: https://support.apple.com/en-ca/HT201295