Customer has to hit the reset button occasionally, the system was running when arrived. Checked the fuel flow and transformer, both ok. Removed the nozzle assembly and there was some carbon on the tip and the tip was black. There was also oil in the bottom of the blast tube, all indicating the the nozzle was not spraying oil correctly into the chamber. Replaced the nozzle and cleaned the assembly. Started and tested. System is operating properly. The furnace is 25+ yrs old. Recommend replacing the system.