heat light on

[RobMatthews]

ok first i know there is loads about this around but here is my question;The heat light has come on and stays on all the time (im not using the car at the moment) now this comes on to show a fault somewhere , am i right so far? so how do i find that fault or faults? the car is single turbo with a hks piggyback ecu so cant look on a pfc commander. Im pretty sure the light came on after the battery was disconnected to be charged if that means anything?anything else you need to know to help me please ask
thanks in advance for the help,rob