Welchala

I have a question I'm hoping you folks can help with. I have been looking over the forum and haven't found quite the answer to my question. I have a Hot Springs Prodigy 2003 running on 120v. IQ 2020 circuit board. The heater stopped heating the hot tub. No visible scorch marks on the heater relay board. Heater works when powered direct, I am heating the tub that way. Should I replace the heater relay board and thermistors for a $150 or is there a way to test them with a volt ohmeter? Or should I have service come out for $150 to check it out? Have new filters, have powered down for 30 seconds. Any advice on what to check for? Thanks