I'm having problems with my 7 year old Mastertemp 400. After much searching I had found a previous thread describing my exact symptoms ,MasterTemp 400 Heater won't stay running, but it was never confirmed if a new control board solved the problem?
The heater seems to be starting normally when the controller calls for heat. The blower spins up with the HEATING led flashing for 5-10seconds before the burner kicks on. However the instant the burner ignites, the SERVICE HEATER led lights up immediately. The burner continues to fire for 5-10seconds before the burner shuts off. The heater is firing - I can hear the whoosh, feel the exhaust heat up, and readout shows 140-150ish when the SPA ON or POOL ON button is held down. The blower runs its cool down timer and shutoff. I found the AGS led is also lit up on the back of the control board.
I jumpered across the AGS switch, HLS switch, and water pressure switch but the same thing still happened on multiple start ups. I also followed the trouble shooting guide in the Pentair manual. I pulled the thermal governor (tstat) and verified it opens in hot water. But it was difficult to remove which indicated the internal bypass may have failed. Pulled the header and yes the bypass had failed so thought I had found the problem. Replaced the internal bypass valve and recovered the missing valve & stem piece from downstream piping but still have same symptoms. The inside of the HX tubes, header, and AGS/HLS probes looked clean with no scale or corrosion.
I don't want to simply start replacing components and hoping for luck or dive into the HX if not necessary so any additional help/comments/insights would be greatly appreciated. Thanks