Response to Good Morning Mr. Harrision, Hope you are doing well. The PO code is in reference to the water temp sensor. The STK0021 would be the correct replacement as long as the wires are hooked up to WS on the control board inside the machine. The item pictured is the Water Pressure Switch & would only be needed if you had FLO on the display with the circulation pump running. Thanks, Robert Brown AquaCal Tech Support

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