Have an older oil-fire furnace with problems. Living in So. Cal. means I can't find anyone to service it. Typical furnace (O.F.F.C.O. Brand probably obsolete, but typical design with good Beckett system), honeywell fan limit controller, White-Rogers controller, etc. The system will start up, ignite and burn, and the fan will turn on (cycle a few times). The system will run for some time (5 - 10 minutes) but will shut down before the thermostat tells it to. I've watched it and the fan controller does not overtemp. The system will trip, the fan will continue on a cool down cycle and I will need to reset to get it to start again. I've replaced the L4064 fan controller and get smoother fan control but still trips. Now I'm going to replace the W-R controller.
Could it be the flame detector shutting down ignition?