Is the heater actually firing for 30 sec and shutting off? or is the fan just running? If your heater fires, it may be something other than the electronics, it would appear the heater is actually working, and you have a problem (physical), that is shutting the heater down (safety switches are saying something is wrong). When you say the temp displays 0, is that the stack flue temp (holding the spa or pool button for 10 sec)? You may have a sooted or scaled exchanger. You could also try checking the SFS setting on the board. It was probably set correctly (obviously) originally, but could have been changed either by a human, or changed itself to a stack flue switch. I have seen where a board actually changed itself, probably through a power outage, lightning strike nearby, or a "brown out". The fact that you are getting a 0 temp reading suggests that this may have happened. If the water temp is in the mid to high 80's, it could be a therm-bypass. Or could be a Fenwall failure, or even an igniter that isn't rectifying. All these things will allow the heater to run for a short time then shut off.
Since im not there, i can't tell you with any certainty that any of these things are the cause. I'm just throwing things out there for you to check.