I have a older Tiger River Bengal model M that will not heat. I bought 2nd hand. I have replaced the heater, circ pump and heater relay board. I have thoroughly cleaned the filter as well as removed possible air locks. The heater is a no fault 6KW so there is no pressure switch (fuse in panel). When I take control panel cover off, htr on is lit red, and lim light is green. There was an ozonator that was installed, but currently not hooked up. I have water flowing into spa as I can see the water moving, however there are no bubbles. If I disconnect hose from circ pump and plug circ pump with my thumb to pressurize, when I reconnect I will get bubbles for 1-2 seconds. Not sure what additional steps to take?