I bought a Sta Rite Max-E-Therm 400HD pool heater in 2017.  Just opened the pool to see how things were running.

Heater fired up just fine, but the return water didn't feel as hot as it used to be and the exahust felt hot.  I checked the temp (held down the Pool On button 5 secs).  It said the temp was 311 degrees.  What should I do next?  It has the Cupro-Nickel heat exchanger.  Concerned about a problem with heat exchange...