Hot Springs Sovereign VIntage 1998 (but with new nofault heater last year)

Green ready light is blinking.

Circulator appears to be working.

After power up (with cold water in - 45 deg) limit ok and heater led are
on, then after about 10 sec. limit ok led goes out, then in a few more seconds the heater led goes out.

Is the hi-limit bad? or the temp sensor? How to test them (ohm ranges available somwhere? thanks