I think I have a bad temp sensor/probe.  It says "HI" in the display, manual says that means water is 114+, it is NOT and has never been that hot.  Cycling the power at the breaker does not reset it .  Looks like a simple probe swap, but the guy at the pool store said there are also two high temp switches to protect against overheating.  The parts are about $44 each, anyone have any experience/ideas, I'd rather not buy all 3 parts, they are special order items.

Thanks

could be a flow problem as a poolman the first thing do is clear all baskets & backwash  or clean filter is this your heaterhttp://www.pentairpool.com/pdfs/MiniMaxNTHeater-DDTC-IG.pdf

if it is I would think it is temp probe figure 5 P9 in RED on drawing

I would also check water chemistry you could be eating up your heater causing probe failure

GOOD LUCK

Baskeets emptied weekly, backflushed last weekend.  

Water parameters monitored weekly.  Heater is about 4-5 years old, last week it worked, this week it claims "HI", and will not cycle.  
I think it's either the "thermistor probe" of one of two "sensor high limits"

That link is my heater.

THanks, that;'s what I figured, but I don't know which sensor fo the three to replace?
I run it once a month all year, every weekend during summer.
Any idea which of hte three probes is bad?  They are $45 each, I'd rather not blow $100 extra than I need to.