I'll start by saying I have read through all the posts on this type of issue that I can find and have tried just about everything noted in them. I have a 4 year old MasterTemp 400 that is very rarely used. This past weekend after cleaning my filter I turned on the heater just to try it out before I start needing it (I live south of Houston and won't really need this for another month). The heater fired up normally and I thought all was well. I finished cleaning up and forgot about the heater being turned on. I remembered it about an hour later and went out to shut it off and found the Service Heater light on. The control panel was locked up and would not let me turn off the heater. I had to turn off the breaker to regain control of the control panel. As soon as the breaker was turned back on the heater started back into its start up cycle. After going thru the normal startup routine the heater again fired up normally, ran for 20 seconds +/, tripped the service heater light and locked up again. I looked at the back of the control board and found the AGS light on.
After going through the cycle a few times I see the following pattern.
1. Turn on the heater (either "pool on" or "spa on"
2. Everything starts normally
3. The instant the gas valve opens and the ignitor fires, the Service Heater Light comes on.
4. The heater continues running normally for approximately 20 seconds, definitely heating, hot exhaust for entire 20 secs,
5. Heater shuts down, AGS light illuminates, panel locks up
What I have tried.
1. Checked thermostat - it works open and closes freely under hot top water. Heater functions exactly as above without thermostat installed at all.
2. Checked bypass valve - appears fine, completely clean
3. Checked internal condition of header - all appears clean
4. Replaced AGS switch - no change in operation
5. Looked at Gas regulator (Itron B42), have not disassembled but vent is clear
6. Flow rate seems fine, discharge into pool and over spillway from spa is at its normal high rate
7. Filter is clean and filter pressure is at its lowest rate as always just after cleaning, so a blockage is unlikely
I would love some suggestions. Thanks in advance.