It appear that the heater will not come on due to pressure.
Booklet says either a filter problem or line blocked or something like that. I seem to recall that last winter just cleaning the filter made all the difference. I could clean what I have and see, but it is cold outside and I am under the weather (something I ate??). When the spa is on, there are a LOT of bubbles in the filter and the returns to the spa, but bubbles are not on. Seems like cavitation.
I am thinking, clean the filter or just buy new carts. (did I mention I feel crummy?). Am I being stupid and lazy? Is there something actually wrong with the heater?