What does it mean when both the red and green lights flash when the spa nears temperature setting. If I power off and back on the heater comes on again, red light only on, until it gets near temperature.
Appreciate the help, Glade

Could be a few things:

1. FIlters may be dirty...take them out and try it without.
2. One of your thermistors may be bad
3. The circ pump may be clogged with debris
4. If you have one of the No-fault heaters with the integrated pressure switches then the pressure switch may be bad.

Try the filters first.

Thanks Dan, No filters installed, fairly good flow from circulator pump + ozone bubbles. I'll bug out the switch and thermistors tomorrow. If the switch is not closing I figured it would not turn on the heater. Is hot spring the only supply for replacement thermistors?