Leaderboard

Sounds like poor regulation of the DC voltage on the board.

These boards have proprietary software which makes them difficult to replace with anything other than original however the board can likely be repaired. I'm not questioning your repairs here but I'm curious about the voltage values of the diodes you replaced at D3 and D7? If there are no error codes then the only thing I can think is that the relays are not receiving the correct coil voltage. Unfortunately, the new relays don't have exposed coil pins like the old ones or you could measure the coil voltage from the front of the board. I'd start with tracing down the coil circuit for the high limit (left and upper relay) as they are both turned on by the same circuit. If the temp sensors are both good these two relays should turn on right after boot i.e. there should be 15VDC across their coils. Addendum: I've heard it mentioned that the internet says to use a 9V battery to test these relays. I can't say that it doesn't work however I don't think it's the best idea because you have a 50% chance of applying power in the opposite polarity than the circuit is designed to work with. Some of the components can probably take this mis application of voltage, but I wouldn't do it to my own board.

Where on the planet are you located? Post photos of the circuit board installed and the schematic on the inside cover and the topside so we can see what you have

When the circ cycles off is the board still sending power to it? The small Laing circ pumps are notorious for cycling on/off when they start to fail.

Do you have a multimeter and know the basics? Test for power at the circuit board where the circ connects. Do you get power to the circ even after is shuts off? testing to see if it is a circ pump issue. Is the spa new to you and has run at your location previously? Post photos of the circuit board and the schematic on inside cover Remove filters. Take them out of the equation. Dirty filters are flow killers