Hot tub wont heat above 70 degrees

[Tacticalterry]

I always find the solution to problems by reading what others post, and never need to post my own. That is not the case this time.
My 2005 (Balboa Parts) Four Winds spa quit working. Trying to restart it, I ruined a relay. Long story short, I replaced the bad heater, soldered two relays for sensors, replaced sensors, and soldered a broken wire on the transformer that I'm pretty sure I broke.
Fired hot tub up and it ran perfect, minus one jet adjusting knob snapped off. Ok so it ran for two days at temp without issue. Got in one night changed temp everything works great.
Go to get in last night and it is at 70 degrees and even though the heater light is on it wont get hotter than 70. A few things to note, it worked fine for a few days, it is 20 degrees outside temp today, and the heat sensors do not appear to be adjustable (as far as I can tell) as they are just the plug in probe type. they are new also.
I have had the tub in FC mode I've had it in f2 and f4 and f6 just to see. Also I made sure its in ST so im at a loss why it stopped getting hot.
Ive had it at 102 before when it was really cold, so I would not think that was the issue. Any thoughts?
Thanks in advance

 

[Isaac-1]

If your tub has a circulation pump have you checked to make sure you are getting good water flow. I had something similar when I first bought my hot tub (used). Where the weak circulation pump was causing the water flow through the heater to trip off on over temperature, yet the water in the tub would not get over the mid 80's. Once I replaced the circulation pump I realized the old one was flowing only a fraction as much water as it should have been. At least on mine the heater should never get so how that you can't touch it.

 

[Tacticalterry]

I wonder if the broken jet adjustment valve could be causing it? I'd say it probably is a flow issue, just don't know why it was working the first couple days. Maybe it was a problem and the cold is just making it more obvious. Well I'm going to order a new valve and put it in this weekend. It broke off to the point I can't turn it at all
Thanks for the thought. Pump will be the next thing I replace then. It may have been what made the heater go out. Well that and age

 

[Bama Rambler]

It could also be a bad high temp cutout. Those things get weak over time and tend to open at a lot lower temp that they're designed to. I'd check that while you're looking at things.

 

[Tacticalterry]

So it appears the bolt holding the 220 plate from heater to board had a crack in it. I opened up the tub and noticed a slight bit of scorching around the bolt.
By this time the tub had completely shut off for safety reasons. Thank goodness.
I sanded the connection down. Checked everything for continuity and put a new one back on.
So far it's running. I'll keep an eye on it as it goes along today. Don't like the fix but as there is 4-5 inches of snow on the ground I don't feel I have much choice.
After running a few minutes with ice warning, it has raised four degrees. I'm hoping this will fix it till I get a new board.
Thanks for the help guys.
This just proves, if there is a problem with ur hot tub, the problem is "anything"!
Thanks again