Hot tub warm, but not heating up to set temp

[EllayAmore]

I have a second hand hot tub that I received as a gift and we got it running a couple months ago. We have had some issues lately. It was running fine for about a month until the control panel began showing only a blinking “ready” and blinking “power.” The jets and light work fine but the temperature began to drop and after some research I assumed it must be the temp sensor. My boyfriend replaced that for me yesterday and at the time the water was warm, but not hot. After 24 hours it is starting to drop in temperature and isn’t nearly as warm. Now the screen is just blinking “ready.” I tried turning the power off at the breaker and turning it back on and it starts up fine and screen looks normal but then after about 10 seconds something clicks and the screen just goes back to blinking “ready.”

It’s in the 30’s here in NH and expecting snow soon, so hoping to fix it before conditions get worse! But not sure what else it could be! Water level is over the jets... is there something else we can do? Maybe an issue with the part replacement?

Thanks in advance for any advice or help!!