Pentair Mastertemp 400 - No error code


New member
May 20, 2011
I have replaced both the thermostat and the thermistor. The heater is still not coming on. It goes through the normal on sequence then displays the correct water temp of 84, while the Service Heater red light remains on. There is no sign of the fan coming on or the ignition firing. The gas valve is turned on and the heater temp is set to 104. There are no error codes. I have cleared all baskets and flushed and recharged the filter. Any ideas? Thanks in advance.


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Welcome to TFP.

Have you looked at the diagnostic LEDs on the back of the board?

What lights are coming on on the back of the board?

The following are the LEDs:

1.....PS (Pressure Switch)
2.....HLS (High Limit Switch)
3.....SFS (Stack Flue Sensor)
4.....AFS (Air Flow Switch)
5.....AGS (Automatic Gas Shutoff).
6.....Service System
9.....Pool On
10...Spa On
11...Service Heater.



New member
May 20, 2011
Thanks for the quick response! I probably replaced two good parts... I will look for those leds tomorrow and go from there.