I have a Sta Rite 400 btu heater. Replaced the manifold with tall the sensors. Replace the circuit board as well. Heater runs and keeps running just fine. After about an hour the temp in the hot tub only went up 6 degrees. It should have been at set temperature by then. Does the new circuit board need to be reset after replacing it?

SOURCE: sta-rite heater Display shows 125 degrees, won't activate heater

sounds like the temp sensor is bad. Sta-rite Heater

SOURCE: Temperature reads 125 degrees.

I am not sure if you have solved this issue, or not, but just in case. The problem is the Thermister, or temp sensor. That will need to be replaced since it is reading the water temp at 125.

The Thermister is located on the manifold, left hand side, between the first two bolts. It is pretty easy to change out and this will correct the problem.

SOURCE: Water Temprature read 126 on heater and will not Fire

This is a problem with your thermister (heat sensor) You will find this on the left side of the water manifold about half way down. I'm not sure but I think it is a small tube that slides into the manifold and if you follow the wires back to the board it just plugs in. The part is reasonable priced for a heater part and very easy to install. Good luck.

SOURCE: Ref: Sta-Rite Max-E 400. Pool temp is about 75

Your temp sensor has gone bad and will need to be replaced. It's a small tube that slides into the water manifold and you can follow the wires back to the board. Just unplug and set a new one in place.

SOURCE: Sta rite Maxe therm heater turns on by itself

These pool heaters have a very helpful and easy to follow chart in their users manual for problems such as this one. There are questions that you will need to answer that you cant answer unless you are looking at the heater. Below is a pdf version of the manual.

http://www.backyardcitypools.com/images/bwp/Max-E-Therm-Manual.pdf