shank

I was recently given a 1995 Sundance Marin (850). I filled it with water and was getting a flashing flo error. I took out and inspected the flow switch which appeared not to be damaged and made sure the lines were clear. I also checked to make sure the circulation pump was clear on each end. It seems that the circulation pump is doing it's job well, however the flo error will not go away which also makes it impossible to for the heater to turn on. I let the spa continue to run and the GFCI trips after about 30 minutes. Any advice?