that light signals aproblem with the coolant system,its either a blockage[ie sediment etc'in pipes]or a v.small leak,usually at a hose connection or drain plug [radiator]causing an airlock in the system,drive for ten minutes then park up over a pit or up [one side]on a high path,with engine running look for leaks,[using a torch]then with engine off,if sediment drain the system[into a large container,and flush with hose,run the engine [with plain water]and reflush,u can buy/use chemical flushes but they can further damage [already corroded pipes/hoses.good luck,Indian.
the indian, April 2009