If the heater fails to ignite after three attempts, the heater will automatically go into Lock Out. This is a safety feature on the heater.
Things for you to look at to see what is causing the problem are the display panel and diagnostic lights. When you turn on the heater, is it reading the correct water temp? If not, the Thermister has gone bad and will need to be replaced. When you try to start the heater, is there an Error Code displayed? Which of the Service Lights are coming on when the heater will not light? If it is the Serivce System, that is usually low flow of water. You may have a dirty filter, skimmer or pump basket full of debris, or a valve not completely open.
If the Service System light is the one coming on, then you will need to take off the top two halves of the heater and turn over the Display Panel. There are diagnostic lights on the back side of the panel. There may be a red light next to any of the following:
AGS, AFS, SFS, HLS, PS or THERMISTOR
If there is a red light next to one of these, then that is what has malfunctioned. If you need an Owner's Manual for the heater, you can find one online at the following link: