Found primary control locked out. Before heating reset I checked nozzle, electrodes, transformer, cad cell eye, all components looked good. Found air filter completely plugged up. Replaced air filter, checked for air in oil line none found. After replacing filter I reset primary control and furnace fired up. Furnace stayed running for 30 minutes at which point I turned unit off and restarted to make sure furnace would fire back up, furnace fired back up once again and stayed running, furnace did not teip. Customer has cleaning in January. System is running normally.