Jandy Lite 2 pool heater FL4 message. I replaced the Ignitor. After first ingition heater ran for 30 minutes. Replaced fuses again and heater ran for 3 hours before blowing again. Suggestions for further diagnostics? Thanks.

So, you replaced part R367100 (Igniter)? This part if very susceptible to failure if touched. I would inspect and/or run a continuity check to make sure it has non cracks. Does it glow red when you attempt to start the heater? Remember, that is the how it ignites the flame, by getting red hot.

If it glows red hot, but does not light then I'd do the following testing in between;

Check that the inline fuses on the igniter aren't blown,

Check if the control board is sending a charge,

Check the flame sensor for gunk build up. Unfortunately to replace the flame sensor look like you have to purchase a whole new burner assembly to get a new flame sensor.

I also believe there is a toggle switch on the front door. It may need to be depressed before the system will ignite.