I have had intermittent issues with my 3yr old Pentair Mastertemp 400. It would occasionally show a temp that was much higher than the actual water temp. If the pool water was say 65D the heater would show a water temp of 115D. After a couple of hours the temp on the heater would correct itself. But because it was reading a temp so high, it wouldn’t fire the burner. Very annoying! I replaced the thermistor, but this didn’t resolve the issue.
After some tinkering (and goolgling) I found the answer:-
I would bleed air out of the Clean & Clear plus filet by opening the manual relief valve on top. This would release the air from the top of the filter and then a jet of water under pressure as all the air is expelled. This jet of water is directed at the heater and was getting the heater manifold wet, as well as the water flow sensor and Thermistor connections. I remove the side panel from the heater and dried the connections. This improved the accuracy of the heater temp display. I then ran a hair dryer on the connections while the heater was on. Hey presto, the numbers dropped to the correct values in 15 seconds!
So if your Pentair Mastertemp is showing the wrong temp, try not getting it wet!