I have a Great Lakes spa, 2004 Manitou model and some time ago I started getting the OH message every time. (it has a digital display)
Without beeing much of a techie, I know few things about electronics so I started some rudimetary troubleshooting combined with engineergin common sense.
After checking OK the sensors, the control panel and the pressure switch (which I do not consider rocket science, but it may prove me wrong in the end) I assumed that the board was faulty (I had no practical way of checking it) so I forked $300 for a new one. I installed a week ago and after turning the spa back on to my dismay I got the same result.

The pump and heater keep going until water temp reaches setpoint. Then the pump stops, but not the heater. The heater keeps going until water gets so hot that it trips the high-limit sensor at it locks the panel.
Basically as soon as I turned the spa on, the heater if on regardless of cycle/temp/etc.

I had encounter much greater technical challenging equipment in my life but this one proves a tough one to crack as I cannot find a reasonable explanation.

Any help or advice would be GREATLY appreciated. Thanks

There are a lot of things in line to stop the heater from getting power. If you're getting to the set point and the pump cuts off, that should open both the thermostat and pressure switch. That would make me think that it would have to be the heater relay, but I assume all your relays are on the new circuit board?

That is correct. All relays are fine even on the old board.
Now, there are only 4 inputs to the board: -temp sensor in the spa / - control panel / - high limit / - pressure switch.

As the board has been replaced whith a new one of the board the culprit would be one of the following:

hi-limit - works as intended - shuts off the spa when water starts boiling in the heater tube
temp sensor - indicates the right temp
control panel - I don't know what would be wrong with it, has only 3 buttons and seems ok
pressure switch - I do not really understand this device- I thouhg it would be kind of OFF/ON switch but heater will not stop even when i disconnect the press swthc.
And after all the effort and frustration I find it close to impossible to get 2 boards with exactly the same problem....

..but again, hottubs are not my forte - this one come with the house so I am new to this field. ..

If the heater doesn't turn off when you disconnect the pressure switch you have a definite board issue.

Man, that really sucks !!!

So, a brand new board that has exactly the same problem as the 4-year one it replaced ?!?!?

What are the chances ?? Should I play the lottery with this kind of odds