Explosive ignition in Hayward AGP Heater

[JoeG]

I have an older H100 model Above Ground Pool Hayward Heater. It has just started making a loud, explosive bang with ignition. You can see the flash inside the cabinet. I have shut it all down and turned off the gas. I am trying to figure out where to start troubleshooting and what to replace first. What is the most likely cause of this kind of explosive ignition?

Thanks,

Joe