Pool and Spa
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The p51 code you are referring to is actually pressure switch 1(ps1) and indicates that the pressure switch did not close to allow voltage to get to heater so you need to check for good flow of water through the heater and if the pressure is good then you have a bad pressure switch and replace it. The pressure switch is usually located on the heater housing. the switch is bad if water is good and looks like a small UFO with two wires on the top of it going into the control box and the wires plug into the circuit board. If you have a clogged pump or a pump that is not working the bladder in the pressure switch will not close and cause this error code. You can test the pressure switch by following the wires to the circuit board and jumping across the two leads on the board to simulate the switch closing and if the heater light comes on you know the switch is bad. Do not let the spa run for any length of time with the switch jumped as this is a fire hazard.