Sta-Rite Max-E-Therm SR400NA pool/spa heater

[EdM]

Tried starting up the heater after a couple of months of not needing it.
It seemed to start up normally, then shut down after not even a minute. The display says r9 and the the temperature reads 126.

After shutting down the unit I removed the thermister and it felt hot. The probe was pitted and looked bad, so I ordered a new one from Amazon.

Installed the new one and turned the unit on. Once again everything looked good. The water temp read eighty something. The heater fired up and lasted less than a minute. Same thing, r9 and the water temp reading 126 degrees. At that time the ohmic value across the thermister was 10 using a cheap meter that only has a 1k scale. Read 6 ohms on the old thermister. Turned the pump back on for water to flow thru the heater and the ohmic value on the new thermister came down to about 4.5 ohms.

I checked the wiring from the circuit board to the thermister, it's good. Each stab on the circuit board that the thermister wires plug into , with the board totally disconnected reads 18 ohms stab to stab. Plugged back in and mounted it's 3 ohms stab to stab. 

Are those readings normal? Is the problem a bad thermister with the new one I bought being bad also? Or, is the circuit board shorted?

When I removed the circuit board, the back side of it that was facing up, was covered in dust and after cleaning it, I noticed that side was wavy looking like it had been wet.

Appreciate any ideas

Thanks

[Pool Clown]

The problem may be the thermostatic by-pass.  If it has ANY corrosion what so ever, it will impede the parts ability to open correctly.