Can't seem to fix flashing power light on HotSprings Vanguard

[dguisinger]

I've got a mid-2000s HotSprings Vanguard hot tub.  This winter it started tripping the high temp lockout every couple of days.

As an initial troubleshooting step, I replaced the filter.  It did not help.
I was advised it was a bad heater, the heater was 7 years old so I ordered and installed a replacement.
The problem continued, so I was advised it was the circulation pump and the circulation pump was 10 years old, so I ordered and installed the replacement.

The problem continues.  Its been too cold out to drain and refill to see if I have an airlock that can be solved by that.  It did warm up enough one day to turn the water on and top off through the filter outlet tube, since repairs caused some water loss.  Turning the water on almost immediately put the tub into a fault, though a google search says this could be a rapid temperature change issue with the sensors and not the actual issue.

I have tried bleeding air out by disconnecting the pump multiple times, I haven't seen/heard any evidence of air coming out.  I have also loosened the pump where you can open it up to look at the impeller to try to let air out that way, all I've been able to detect is water coming out of it.

The tub does gurgle and splutter when the circulation motor is going.  This happens with the new one, it happened with the last one...I actually don't remember a time it was ever silent, so maybe there is some level of gurgling that doesn't indicate an air lock... However, I discovered last week that the gurgling does go away if I turn the "waterfall" feature on, and then all you hear is the silent hum of the circ pump.  Doesn't matter how long I run the waterfall, as soon as I close it off, the gurgling sound comes back.

The main tubing has turned completely yellow so I can't see inside if there is any air trapped or debris.  If there was debris, I never found anything inside my old pump which looked clean when I took it apart.  I did have a hot tub blanket where many of the plastic bubbles started to peel off and I used to find them floating around... but I've not found evidence of them inside the plumbing as of yet.

Last year I had to do a massive teardown to fix a $0.50 cent leak and then put hundreds of dollars of foam back in, and then had to replace the heater relay board shortly after.  With this tub approaching 20 years old, I really didn't want to spend anymore money, and most certainly don't want to throw even more parts at it after doing both the heater and pump.

Any ideas on what I could be missing?

[poetical]

I had this issue and it turned out there was slushy ice in the lines around the tub causing low circulation and the heater to get too hot.

i was still able to run the main pumps which I did for an hour and that in itself can surprisingly warm the water more than you’d think.

i also got a 100w incandescent bulb (didn’t have a ceramic heater), and left than in the compartment overnight.

it than ran just fine the next day. If you’ve had a long period of weather at temps around freezing, this could be your issue?