Solenoid Valves : Your shower requires low water pressure the ensure the Solenoid Valve is closed. A faulty Solenoid Valve will not pump water into the shower and you maybe without water.
Thermal Cut Out (TCO) Switch: If your TCO is faulty you will experience no hot water as the TCO s used as a sensor for hot water, and if your water is to hot the TCO will shut down the hot water to avoid you getting scalded.
