CLK error message
The manual is vague as to to solution. It says something about the clock not being set, but doesn't mention how to set it, also says something about a fireman swich.
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Answers & Comments
CLK means the firemans switch circuit is open. Inside a timer, you will see a litle black switch next to the time dial and there should be two wires connected to it that are from the heater. If that is how your is set up, the switch is not closing for some reason. You can bypass it inside the heater.