'99 Hot Springs Sovereign, running 110V. Has a fairly new (4 months)replacement Watkins heater (PDR, 4kw version).

After working perfectly for 4 months, I came home from a long weekend with my red power light blinking. Our power went out at some point while we were gone. Not sure how long it was down, but the water temp was in the 80's. I unplugged and when I plugged it back in everything seemed back to normal and the light stopped blinking. I was able to run jets, light, circ pump seems to be running, and eventually the green "ready" light came on. Only problem is the temp is only about 90 degrees, so not really "ready". I had it set for 104. It holds temp at 90, but doesn't get any hotter.

Any thoughts?

1st thing that comes to mind is a bad thermistor. Checks Ohms across both thermistors- see if there's a significant difference (set meter to 20k if not an auto-range)
Likely just a bad control thermistor-very inexpensive compared to that heater!

Once again, right on with advise. I replaced both thermistors this weekend and I'm back in business. I initially thought something was wrong because I didn't see any difference for the first 24 hours, but before I posted again I checked last night and I was at 104 degrees (48 hours later). Don't know why nothing happened the 1st day, but it looks like I'm good now. Thanks again.