How to Diagnose Oil Furnace Ignitor Problems


An oil furnace operates with a high-voltage spark ignitor adjacent to a nozzle. Air and oil under high pressure flow through the nozzle to combust under the spark, igniting the oil burner. If the furnace will not light, try troubleshooting the ignitor electrode and fuel nozzle for common problems that you may be able to correct yourself without calling for furnace repairs. Wear work gloves while working around sharp metal edges.

Things You'll Need

  • Work gloves
  • Screwdriver set
  • Old rags
  • Ohmmeter
  • Shut down the furnace at the thermostat control and wait for the unit to cool.

  • Close the valve in the oil line connected to the furnace. The knob turns to the right to shut off the fuel oil. Cut off the circuit breaker for the wire to the furnace that powers the ignition system.

  • Remove the screws holding the access panel to the metal cabinet. Take off the panel.

  • Take off the cowling on the end of the blast cone, which is positioned below the burner. The cowling unscrews on some furnace models and pulls straight off on others. The ignitor electrode and fuel nozzle are now exposed.

  • Wipe the opening of the fuel nozzle with a rag to remove any clogs.

  • Secure the wires from the back of the ignitor electrode to the two contacts on the ignition transformer, which is box shaped. The transformer produces the high voltage for the spark on the electrode tip.

  • Turn on the circuit breaker.

  • Press the red button on the outside of the oil furnace while watching the ignitor tip. Keep your hands clear of the ignitor at all times while testing. If you do not see a spark, continue troubleshooting.

  • Shut off the circuit breaker.

  • Pull off the two wires on the ignitor transformer. Each wire is attached to a clip that slips on the transformer contacts.

  • Touch the two rods on the ohmmeter to the two wires. A zero reading means the ignitor electrode is defective and must be replaced. If the ohmmeter displays any level of resistance, which will vary with the furnace model, continue troubleshooting.

  • Touch the two ohmmeter rods to the contacts on the transformer. If the meter registers zero, the transformer is faulty.

Tips & Warnings

  • If the electrode tip is defective, a new electrode must be mounted in the same position as the original part. The spark from the electrode must make contact with the air-oil vapor from the nozzle at a precise point. Do not bend or alter the mounting bracket for the electrode.

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