Marquis spa Costal series won’t heat unusual circumstances

[Electromechanics]

Hi guys,
I just bought a home with a Marquis Costal series Spa (see attached pictures).
My issue is with the heating, I set it to 104F but it only heats to ~94F. In addition, it cools down while we use it to below 80F and the heater does not activate.
I have checked the following:
1. Heater relays are ok (resistance across all relay coils are equal)
2. Heating element Ok (resistance computes to rated Kw)
3. Temp sensor ok (reads water temperature and displays on top side controller)
4. Pressure switch ok (I also bridged it out for testing)
5. Pumps run fine and flow is good
6. Heating element housing has a sensor, but I can’t check or test

Does anyone know how to test the heating element sensor or have any suggestions?
I suspect it might be a setting on the top side controller but I can’t find a manuals.

 

[RDspaguy]

coils are equal)

Which means the coils are ok, not the relay. The most common circuit board failure is burned out contacts.

Temp sensor ok

You took an ohm reading in the 20k range?

Pressure switch ok (

The system is smart enough to know if it's closed when it should be open or vice-versa.

Heating element housing has a sensor, but I can’t check or test

You test it the same way you do the temp sensor, with an ohm meter.
Are there any errors on the display?
Is the spa wired for 120v or 240v? Verify incoming voltage.
Check voltage at the heater connection on the circuit board. Test from terminal to terminal, not terminal to neutral or ground.
Test sensors with ohm meter and post results.
Post better pics of the circuit board, and check the back for burns or dark spots.