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Sundance Circuit Board Guide (6600-726 vs. 6600-720) The "Hidden" Difference: Series 3 vs. Series 5 The Sundance 6600-726 and Sundance 6600-720 boards look identical, but they use different software logic. Installing the wrong one can prevent your heater or circulation pump from running. Check your EPROM Chip: Look at the small white sticker on the main processor of your old board. Chip starts with a "3": Use the 6600-726. This supports spas with a circulation pump (like the Certa or Chelsee). …