I have had this spa for about 5 years now, and it suits our needs perfectly. We have a problem this year. When we run it on low, it heats past 104 and eventually goes to "OH" I clear it by unplugging (110) it and letting it sit for a while and then it will run again, but right back up over 104 and eventually back to "OH" So I'm thinking High Limit/Freeze sensor.

I also have another problem where the breaker will trip when the jets are on high for about 3-4 minutes. I'm not sure what to make of this one - it does not trip the GCFI outlet the spa is plugged into on the deck, just the breaker in the garage that the spa is on (all by itself) I have not replaced the breaker since I installed the tub, why would it all of a sudden be tripping? Are these items related? Could the breaker be bad? I believe it is just a 15 AMP breaker - maybe 20, but I don't think so.

Yes they are related, you are heating while your pump is on "High" which is too much amperage for the circuit, tripping the breaker. You heater relay seems to be stuck shut or a bad circuit board. I don't think it is your sensors.

What does my heater relay look like and how can I "un-stuck" it?

I'd like to do this myself mainly because almost every spa technician and repair company is always weeks if not months from getting you on their schedule.

Thanks for your help.

if you do an electrical test, turn down the heat and test the terminals on the heater (one meter lead on each terminal, both at the same time) to see if there is voltage. What does it look like, the contacts are closed even when the heat is turned down, or power is turned off.