Can someone give me some advice on my Max e therm 333. I can get the heater to fire but shuts down after two minutes. I sometimes get a AGS error. I have bypassed the ags but still get AGS error and unit shuts off. Changes the AGS anyway and still get AGS. Put new PC board and still get AGS and unit turns off. It goes from before shutting down the lights go between service heater to service system and then eventually to service heater and turns off. I have changed most of the easy to change components/sensors but still turns off. No obvious flow problems and pressure seems good. Any ideas ?