The saga goes on. Replaced the top side control panel - and the relay clicking stopped. Also replaced the heater while I was at it, since the old heater had a tiny leak around one of the temp sensors.
Filled the tub, turned it on, and it behaved normally for several hours - pump running fine, water coming out of jets, temp coming up. Then over the course of about 4-5 hours I had 3 occurrences of "Dr" or "Dry" messages, indicating a dry heater. Checked all of the usual culprits, found nothing wrong, and once again the tub started running normally. Then last night had 2 occurrences of "PR" messages.
This morning I kept a careful watch on the tub for about 20 minutes. In that time, it went thru 3 of the following cycles:
1. Message "24" - which normally only shows when tub is first powered on at the GFCI breaker
2. Message "PR" after a few seconds (priming)
3. After priming complete, message "--F" (trying to establish water temp)
4. After ~1 minute, temp shows as 102
5. Almost immediately cycle restarts and goes back to "24"
Could this be caused by a defective temp sensor on the new heater?
Or is the motherboard faulty?
Other ideas?
Thanks for your help