Oil Furnace Ignitor Troubleshooting and Repair Guide

The following guide helps diagnose, test, and repair common oil furnace ignitor problems, highlighting safe procedures, typical failure modes, and practical fixes for homeowners and maintenance technicians. Oil Furnace Ignitor Troubleshooting is explained step-by-step to speed repairs and avoid unnecessary service calls.

Symptom	Possible Cause	Quick Action
No spark or glow	Burned ignitor, failed transformer, wiring fault	Visual inspect, test for voltage, replace ignitor
Intermittent ignition	Loose connections, weak transformer, fouled electrodes	Tighten wiring, clean electrodes, test continuity
Burner locks out	Cad cell failure, flame out, oil supply issue	Test cad cell, check fuel pump and nozzle

How Oil Furnace Ignitors Work

An oil furnace uses ignition systems to ignite atomized fuel from the nozzle. Common ignitor types are glow plugs (hot surface ignitors) and high-voltage spark electrodes. The ignition control and transformer supply power, while a cadmium sulfide photocell (cad cell) or flame sensor confirms flame presence and prevents lockout.

Safety First: Preparations Before Troubleshooting

Before any inspection, turn off power to the furnace at the breaker and close the fuel shutoff valve when applicable. Allow components to cool. Use insulated tools and a multimeter rated for household use. If unsure about working with high voltage or oil systems, contact a licensed technician.

Tools And Parts Needed For Oil Furnace Ignitor Troubleshooting

• Multimeter (volts, ohms)
• Insulated screwdrivers and nut drivers
• Wire brush and contact cleaner
• Replacement ignitor or electrode kit
• Replacement cad cell or flame sensor (if applicable)
• Protective gloves and safety glasses

Common Symptoms And What They Indicate

No Ignition Or No Spark

If the furnace attempts to start but there is no glow or spark, likely causes include a failed ignitor, broken wires, or a bad transformer. Visual checks often reveal a visibly cracked glow plug or broken electrode tip.

Intermittent Ignition

Intermittent startup usually indicates loose electrical connections, a weakening transformer, or a partially fouled electrode. Vibration and thermal cycling can loosen terminals over time.

Ignitor Glows But Burner Fails To Stay Lit

If the ignitor functions but the burner shuts down or locks out, suspect fuel delivery problems, nozzle clogging, or a faulty cad cell that is not detecting flame. Air in the fuel line or weak pump pressure can also cause flame failure.

Step-By-Step Diagnostic Procedure

1. Visual Inspection

Inspect ignitor and electrode for physical damage, carbon buildup, or melted tips. Check for oil smell or soot, which can indicate incomplete combustion or nozzle problems. Look for loose or corroded wiring and damaged insulation.

2. Check For Power And Control Signals

Restore power and monitor the ignition sequence safely. Use a multimeter to verify that the ignition transformer receives the correct control voltage from the primary control (often 24 VAC or a low-voltage control). If the control sends power but the transformer output is missing, the transformer likely needs replacement.

3. Test The Ignitor Itself

For hot surface ignitors measure resistance with the multimeter. Typical glow plug resistance ranges from 1 to 10 ohms depending on design. An open reading indicates failure. For spark electrodes check for continuity and inspect ceramic insulation. Replace if cracked or burned.

4. Measure Transformer Output

Measure secondary high-voltage output for spark systems only and only if qualified. For many oil burner transformers the HV is in the thousands of volts; technicians typically observe spark visually rather than measuring direct high voltage. Low or no output despite correct input indicates a failed transformer.

5. Inspect And Test The Cad Cell Or Flame Sensor

The cad cell is a photoresistor that detects flame and prevents lockout. Measure its resistance in dark (high resistance) and when flame present (low resistance). A cad cell out of specified range will not sense flame reliably. Ensure wiring polarity and secure mount.

6. Verify Fuel Delivery And Nozzle Condition

Even with a functioning ignitor, a clogged or worn nozzle, weak pump pressure, or air in the line will prevent sustained ignition. Check nozzle spray pattern, pump pressure, and filter condition. Replace nozzle every service interval or when atomization is poor.

Testing Values And Expected Ranges

Component	Test	Expected Range
Hot Surface Ignitor	Resistance (cold)	Approx. 1–10 Ω (manufacturer-specific)
Cad Cell	Dark vs Flame resistance	Dark: High MΩ; Flame: 3–30 kΩ depending on sensor
Low-Voltage Control	Control Signal	Typically 24 VAC control circuit

How To Replace An Ignitor Or Electrode

Removing The Old Ignitor

Turn off electrical power and fuel. Allow the furnace to cool. Disconnect wires and remove mounting screws. Note electrode spacing and orientation for proper spark gap and placement.

Installing A New Unit

Install the replacement ignitor or electrode using manufacturer-recommended torque and gap specifications. For spark electrodes set the gap as specified (often 0.030–0.050 inch). Reattach wires, restore power, and observe the ignition cycle to confirm correct operation.

Maintenance Tips To Prevent Ignitor Failures

• Schedule annual oil burner service that includes nozzle change and electrode cleaning.
• Replace the ignitor based on runtime or at first sign of cracking or reduced performance.
• Keep the burner compartment clean and free of soot, rust, and oil leaks.
• Use quality oil filters and routinely change fuel filters to prevent nozzle clogging.

When To Call A Professional Technician

Call a licensed oil burner technician if the issue involves high-voltage transformer testing, persistent flame instability after basic checks, suspected pump or nozzle replacement, or fuel line repairs. Professional service ensures compliance with safety codes and reliable performance.

Cost Considerations And Typical Part Prices

Replacement ignitors range from $20 to $150 depending on type and OEM. Cad cells cost about $15 to $60. Service calls for diagnostics and repair typically range from $100 to $300, excluding parts. Prices vary by region and technician.

Common Misdiagnoses And How To Avoid Them

Assuming an ignitor is bad when the actual cause is fuel-related wastes time and money. Always verify fuel pressure, nozzle spray pattern, and filter condition before replacing ignition components. Use measured test results rather than only visual inspection when possible.

Quick Troubleshooting Checklist For Technicians

1. Confirm symptoms and error codes from the burner control.
2. Perform visual inspection for damage and soot.
3. Check control voltage and transformer input.
4. Measure ignitor resistance or visually confirm spark/glow.
5. Test cad cell and confirm flame detection wiring.
6. Verify nozzle spray pattern, pump pressure, and filters.
7. Replace faulty components and retest full system cycle.

Resources And Manufacturer Guidance

Consult the furnace or burner manufacturer manual for model-specific ignitor part numbers, resistance specs, and recommended gaps. Manufacturer technical bulletins and OEM replacement instructions are reliable sources for correct installation and troubleshooting data.

Summary: Key Points For Effective Oil Furnace Ignitor Troubleshooting

• Start with safety: power off and fuel isolated before any disassembly.
• Follow a logical diagnostic path: visual, electrical, then fuel system checks.
• Use a multimeter to confirm control voltages and ignitor resistance.
• Check cad cell operation to rule out false lockouts.
• Perform regular maintenance to prevent ignition failures and extend component life.