After a refill my Hot Springs Solana SX hot tub does not heat. Repair man confirmed I need to replace both thermistor sensor and high limit temp sensor. This corresponds to the error message that is on the control panel. I have the replacement sensors. Not being a very good DIY person but knowing how to access them, should I attempt the replacements on my own? If I do, should I drain the hot tub first? Any additional advice is most welcome. Thanks.