My pentair spa won't get above 88 degrees - Pentair MASTER TEMP HEATER 400 000 BTU NG LOW NOX HD EXCHANGER

SOURCE: minimax pool heater cuts out at 91 degrees when remotely set for

Check the temp reading on the heater. It is probably set at 91...heater control should be set to 104

SOURCE: Pentair Mastetemp Pool and Spa Heater will heat to

This could be a number of things. Most often it will be the internal by-pass. You find the assembly inside the water header, it controls water flow through the heater. When it goes bad it can open at a low temp and keep the water from going through the heater.
Also you will want to check the temp probe on the side of the header. If this is bad it will usually show up on the temp display though.

SOURCE: Pentair 400 2 yrs old won't heat spa to 98 degrees

This could be a number of things. Most often it will be the internal by-pass. You find the assembly inside the water header, it controls water flow through the heater. When it goes bad it can open at a low temp and keep the water from going through the heater.
Also you will want to check the temp probe on the side of the header. If this is bad it will usually show up on the temp display though.

SOURCE: have pentair easy touch controls just installed

Go to "calibrate" on your new unit and lower the spa "set at " temp a little