Hello! I recently bought a used Calspa hot tub. First night it worked great! Day 2 its not heating.. The heat light is on and all jets are working.

See if this helps, (https://spacare.com/spawillnotheat.aspx)or get a manual for your spa, or better yet, call a local service provider. As you see the trouble shooting is a tedious process requiring tools and some experience. Start with having your make and model number handy.