The mod is detecting a difference in the "now" resistance and the old resistance.
Let your mod sit unused for 20 minutes or so, then unsrew the atomiser. Press fire button to get "no coil detected". Screw in cold atomiser. press fire again, and press "new".
This sets the cold resistance as your new base resistance. If you keep getting an error, let us know, maybe the coil is loose or an error is occuring with your mod.
Hope this helps!