I have a spa with Balboa Vs501Z unit I installed about 4 years ago along with new circ pump. Power surges are common with bad winter weather, Spa started throwing GFI breaker, replaced the heater element and cured that problem. Then on start up, all goes well, priming, etc until heater switched on the spa shuts down after 5 seconds or so.
Error code I get a Dr code. Tried powering down multiple times no success. No spa techs where I live, remote location, so I have made many repairs. I am inclined to repalce the circuit board next.
I have excellant water flow with no air bubbles.
Just repaced both sensors.