Example: I power on my Pentair 400 and it shows water temperature 84, the heater starts up and the water temp rises quickly (3 min) to heater setting 90 and then cycles on and off every few minutes. No error codes.
It sound slike you do not have enough water going through the heater... Is there a bypass valve that you need to turn to get more water through the heater? I would say that is it... Could you post a picture of your pool equipment pad so we can see what you have
Also please fill out you signature so we know what equipment we are working with, kind of like mine down here.. It helps us