One of two issues here, 1. low water pressure, either blocked filter ( remove and clean, always put it back), airlock, blockage in pipework, someone has turned off a valve etc.
2/ the over temperature switch on the heater has failed to close again after operating. check with a meter to see if there is a circuit across the pins. the difference will be that in the first instance - low water pressure- when the shower is first turned on after a cool down period, the overheat light is not on then comes on very shortly after, i.e. turn off the shower go do something for an hour and try it, this is a water issue, or the light is always on even when you come to the shower first thing in the morning, that's the overheat switch, easily got and cheap to replace. ALWAYS turn the shower off at the fuse board AND the local switch before you open the covers!! Please consider getting a professional in.
Kenny
January 2017
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