**Possibly fixed** I have been dumbfounded by this same issue. Aqualink remote works all lights and pool/spa modes except heat but has been fine for years. Once I got it to come on but wouldn’t go above 99 degrees even if iOnly way I could get heat was manually adjusting temp in the heat unit itself. Even my pool builder couldn’t understand. After reading this thread and trying some trouble shooting, I may have got it back to running sad normal.
To start, I never messed with the buttons on my Jandy JXI heater unit before this not paid attention to what the screen ever read before. While experiencing my issue, screen read “gas heater off” and not the “Remote Tstat “ i understand it was suppose to show. All steps to validate the settings in the menu of the heater unit showed remote tstat selected...so why wasn’t it working?
The lights are the key. On the spa side there are two lights (green and red). Red means heat is activated and green I’ll get to in a second. During my problem where the Aqualink wouldn’t control heat, neither light was on. However, once I pressed the SPA button, the green light came on. Now the Aqualink pda was in control of the heater but I still couldn’t get the heat above 99 degrees.
What worked for me was manually adjusting the temp in the heat unit to something high (I used 102). Then ensure the SPA showed the green light. My guess is the temp setting that I was adjusting on the heat unit was capping the temperature not to exceed and ignoring the pda setting when wanted a temp above 99. By increasing the unit temp, now the system would let me adjust on the Aqualink pda and allow a higher temp. I tested at 100 and 102 on the pda. Using a thermometer, the water temp adjusted with the pda after following those steps I took.
Hope this helps someone, now I get to cancel my service call.