I have a 5 year-old Monessen Natural Blaze NB24N8C natural gas vent free setup. It ignites and runs for about 5 minutes, shuts off, then tries to relight itself before shutting down completely. I see a green light blinking every 2 seconds on the command center. This error code indicates a pilot lockout, I believe. I thoroughly cleaned the flame sensor, but that did not help. It ignites fine, but will not run very long before shutting down. It's operated perfectly the last 5 winters and has not been adjusted/damaged. What part is most likely needed to fix this? Thanks.