iMac 21.5 late 2013 idle fan speed
A fan speed of 1400RPM really is nothing to worry about. And your model iMac does not require an external thermal sensor for an ssd upgrade, so that shouldn't be an issue for you.

My first step would be to try resetting your System Management Controller. To do this:


Shut down your computer
Unplug it from power
Wait 15 seconds
Plug the power cord back in
Wait for an additional 5 seconds
Press the power button to turn it back on


Software option

You can also try installing SSD Fan Control to see if that resolves your issue. However, I would only do that if the SMC reset does not resolve your issue.