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I have an ST699 control unit on my central heating system (some 15 years old) which has developed a strange fault. After running normally, when the required room temperature is reached it cuts out as usual but immediately loses electrical power, with both the time display and the red lights going out. I have then to switch the power supply off and turn it on, whereupon the unit fires up. Once again, when it cuts out it loses power and I repeatedly have to go through the same procedure. Is this a known fault and, if so, can it be rectified or do I need to replace the unit?