How Many Ohms Should a Furnace Ignitor Have

Introduction: A homeowner or technician often needs to know the expected resistance of a furnace ignitor to diagnose ignition problems. Typical hot-surface ignitors (HSIs) and other types have characteristic ohm ranges, but values vary by material and manufacturer. This article explains typical resistance ranges, how to test an ignitor correctly, common failure signs, and practical replacement guidance.

Ignitor Type	Typical Cold Resistance Range (Ohms)	Notes
Silicon Carbide HSI	40–90 Ω	Common in older furnaces; variances by part
Silicon Nitride HSI	80–300 Ω	More durable; higher cold resistance
Flame Sensor (Not Ignitor)	Thousands to Megaohms	Different function; not measured the same way
Glow Bar / Rod Ignitor	Varies Widely	Often specified by OEM; check manual

What Is A Furnace Ignitor And Why Resistance Matters

An ignitor is the component that creates the heat or spark to ignite the furnace’s gas. Most modern furnaces use hot-surface ignitors (HSIs) made from silicon carbide or silicon nitride that glow red-hot to ignite the burner. The electrical resistance of an ignitor is a key diagnostic: it indicates the material condition and whether the element is intact.

Typical Resistance Ranges For Common Ignitors

Resistance values differ by material, design, and whether the reading is taken cold or hot. Cold resistance (room temperature) is what technicians typically measure with a multimeter. For HSIs, expect ranges roughly as follows:

• Silicon Carbide HSIs: ~40–90 ohms cold.
• Silicon Nitride HSIs: ~80–300 ohms cold; many modern furnaces use this material for longevity.
• Other Glow Rods or Rod-Style Ignitors: Specified by the manufacturer; resistance may be outside the HSI ranges.

These ranges are guidelines. OEM documentation is the authoritative source for a specific furnace model.

How To Test An Ignitor With A Multimeter

Testing an ignitor is straightforward but requires care to avoid damage. Follow these steps:

1. Turn power off to the furnace at the breaker and remove the furnace access panel.
2. Locate and unplug the ignitor harness to isolate the element from the control board.
3. Set a digital multimeter to the ohms (Ω) range; use 2000Ω or 200Ω range if manual selection is required.
4. Place meter leads on the two ignitor terminals and read the resistance.

Interpreting the reading: A value within the expected OEM range suggests the ignitor element is likely intact. A reading of open circuit (OL) or extremely high resistance indicates a broken filament and a failed ignitor. A reading far below the expected range could indicate a short or damaged element. Always compare to the furnace’s service manual.

Cold Versus Hot Resistance — Why It Matters

Ignitor resistance changes as temperature rises. HSIs typically have a lower resistance when hot, which is normal. Meter tests are performed cold for safety and consistency. A cold resistance within range does not guarantee proper operation under load, but an open or clearly out-of-range cold reading is a reliable sign of failure.

Common Symptoms Of A Failing Ignitor

• Furnace attempts to ignite but burners do not light.
• Long ignition delays or repeated ignition cycles before success.
• Visible cracks, blackened areas, or warping on the ignitor element.
• Short cycling of furnace or error codes on the control board related to ignition failure.

When these symptoms appear, measuring the ignitor resistance is one of the first diagnostic steps.

When To Replace The Ignitor

Replace the ignitor when the resistance measurement is outside the manufacturer’s acceptable range, the element is physically damaged, or ignition attempts consistently fail. Because ignitors become more fragile with age, proactive replacement during a heating season is often advisable if failure is imminent.

Best Practices For Safe Testing And Replacement

• Always shut off power to the furnace before touching the ignitor or wiring.
• Handle HSIs carefully; they are brittle and can crack from fingertips or sudden force.
• Disconnect the ignitor harness from the control board before measuring to avoid false readings.
• Wear gloves and eye protection when working near the burner chamber due to sharp edges and soot.
• Match the replacement part to the OEM part number or exact specifications (voltage, shape, mounting, and resistance range).

Why An Ignitor Can Fail Early

Common causes of early ignitor failure include thermal cycling stress, vibration, contamination from oil or dust, electrical surges, and improper handling. Moisture intrusion or corrosion at terminals can also cause higher resistance and intermittent faults. Using the correct part and ensuring proper installation reduces premature failure risk.

Interpreting Odd or Borderline Resistance Readings

A borderline reading close to the upper or lower spec limit can be ambiguous. If an ignitor reads slightly outside the recommended range but looks undamaged, consider the following actions:

• Retest with a different meter to rule out tool error.
• Check harness connectors and wiring for corrosion or looseness causing added resistance.
• Perform an operational test under supervision (power on, watch glow) only if safe and the technician is trained.
• Replace if issues persist—replacing a marginal ignitor before total failure prevents mid-season outages.

Differences Between Ignitor Types And Their Testing

HSIs are resistive elements that glow; they are tested with ohms measurement. Flame sensors and spark igniters are different components: spark systems use electrodes and ignition modules, while flame sensors detect ionization currents and are measured differently. Never confuse these parts when diagnosing ignition issues.

How To Find The Correct Resistance Specification

Always consult the furnace’s service manual or the ignitor part datasheet for the exact cold resistance specification. Manufacturer websites, OEM parts diagrams, and HVAC technical resources provide the precise values. When in doubt, note the model number, serial number, and part number and search the manufacturer database.

Replacement Costs And DIY Considerations

Replacement ignitors typically cost between $30 and $90 for most residential furnaces, depending on type and OEM. Labor costs for professional replacement range from $80 to $200. Many homeowners with mechanical aptitude can replace a plug-in ignitor safely, but caution is advised due to fragile elements and electrical hazards. If uncertain, hiring a licensed HVAC technician is recommended.

Troubleshooting Checklist For Ignition Problems

1. Verify power and thermostat calls for heat.
2. Inspect error codes on the furnace control board.
3. Visually inspect ignitor for cracks or discoloration.
4. Measure cold resistance with the harness unplugged.
5. Check wiring, connectors, and control board outputs.
6. Replace ignitor if open, out of specified range, or physically damaged.
7. After replacement, observe ignition cycles to confirm repair.

When To Call A Professional

Call a licensed HVAC technician if the homeowner is uncomfortable working inside the furnace, if wiring or the control board appears damaged, or if the replacement does not resolve ignition problems. Professionals can also check gas pressure, valve operation, and safety controls that affect ignition reliability.

Key Takeaways And Practical Tips

• Measure Cold Resistance: Always measure ignitor resistance with the furnace powered off and the ignitor disconnected.
• Expect Manufacturer Ranges: Silicon carbide HSIs often read ~40–90 Ω; silicon nitride HSIs often read ~80–300 Ω; verify with OEM specs.
• Open Circuit Means Replace: An open reading (no continuity) indicates a failed ignitor.
• Handle Carefully: Ignitors are brittle; avoid touching the element or dropping it.
• Replace Proactively: Replace marginal ignitors to prevent unexpected heating outages.

Accurate resistance testing, correct parts selection, and safe handling provide the best chance of a reliable furnace ignition system. For specific resistance values, consult the furnace owner manual or part specifications to ensure proper diagnostics and replacement.