I have a Jacuzzi Z140, with a Balboa H276 control board. I started getting a
blinking status light indicating a problem, and the spa would not operate.
After a lot of troubleshooting, we determined that the pressure switch was at
fault. This was repaired and everything was put back together. The jets
operate normally and it goes through its filter routine every 12 hours. The
status/heat light no longer blinks. The problem is, the heater does not seem
to work. The status/heat light (that indicates when the heater is on) will no
longer illuminate, and indeed the heater doesn't come on. The LED does still function,
it comes on briefly when the breaker is turned on. The thermostat
appears to be functioning properly, because when I turn the dial to hotter, the
jets will come on low to work with the heater. After watching the relays click
on and off, we determined that all of the sensors and equipment seem to be
operational. We even made sure the relays were all functional by measuring
their resistance and tripping them with a 9v battery when the circuit board was
out. The heater itself seems to also function ok. We once closed its relay
manually and could hear the water start to sizzle in the heater. It does not
seem like any one item is faulty. I'm left thinking that the logic in the
circuit board has "broken", and it simply never agrees to trip the relay to
turn the heater on. Is this common?

I have basically the same problem, I replaced the pressure switch and now the tub will "sometimes" turn the heat light on but only for 5-10 mintues, then will go out and will not come back on unless the tub sits off for quite a while. I have posted my problem on the site, it seems it has stumped the experts. Have you had any luck....???

Gentlemen,
The concept is simple enough, PLC calls for heat, pressure switch "sees"
low speed come on, and heater relay throws to a resistive load element.
Thermistor value rises to it's 104 Deg value, and cancels call for heat. Your remote spa side controller has a potentiometer (rotary resistive dial) that may be the problem. Your board may also be the problem. If the software, or hardware gets goofy on the board, all bets are off. I have replaced many a H276 board for various reasons. Unfortunately, if you do not have a blinking heater indicator light ( I. E. halt on error ), then its board and spaside time.
The things to check sounds as though they have already been addressed.
If the pressure switch isn't sticking, and there is adequate water flow, then it should heat. Check pump cord connections, and of course element resistance. I know all of the above is not what you wanted to hear.

Turned out it was the pressure sensor. I was doing some diagnosing previously and I thought that the 2 wires to the sensor were open when all is good, actually the switch should be *closed*.