The SERVICE SYSTEM light indicates that there is insufficient
water flow to the heater. If the pump is operating, this
usually indicates that the filter and/or skimmers should be
cleaned (some filters may require backwashing). If the light
remains on after the filter/skimmers have been serviced,
the system should be checked by a qualified service technician. (from the manual))
If you clean the filters, and know the pump is pushing enough water, replace the water pressure switch, or test it first by jumping the 2 switch wires.
If this still is not the issue everything else is way to complicated to relay through an online media. It would be best to call a certified PENTAIR technician to solve this issue. As these heaters are very proprietary and their codes can mean many things, make sure you get somebody with experience on this model.
Below is a link to find a tech in your area.
http://www.pentairpartners.com/dealerlocator/dealerlocatorlist.aspx
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