Ignitor and Flame Sensor Replacement

Purpose

This standing instruction makes an ignitor or flame sensor replacement end with two measured numbers and a named cause of death, so the part that failed does not take its replacement with it inside the season.

These are the two cheapest parts on a furnace and they drive an outsized share of repeat visits, because both fail as messengers. An ignitor that cracked at eleven months was usually run by a board putting the wrong family voltage across it. A flame sensor that would not hold flame is often reporting a bad ground path or a burner drifting rich, not its own oxidation.

Safety actions that gate the work

• Close the appliance gas shutoff before any part of the burner assembly is disturbed, and leave it closed until step 6, which is the first step that requires flame. The sensor sits in the flame path and the ignitor sits in front of the burner ports.
• Prove the control circuit dead before opening the control compartment, proved on a known live source before and after per NFPA 70E-2021, 120.5, with work practices at 29 CFR 1910.333(b)(2). The ignitor circuit carries 120 V at the leads during trial, on a furnace whose door switch a tech has often already defeated with a magnet.
• A hot surface ignitor stays hot for minutes after the call and shatters when touched. Give it four minutes, confirm it has stopped glowing, and wear eye protection removing one.
• Vacuuming a sooted burner compartment lofts combustion particulate into your breathing zone. Use a HEPA-filtered vacuum at the deposit rather than compressed air, and where the deposit visibly clouds, respiratory protection under a written program per 29 CFR 1910.134. Do not scrape or abrade unidentified cabinet liner or door gasket for access: that is a separate scope under 29 CFR 1926.1101 for repair work, or 1910.1001 in general industry.

Scope

Covers hot surface ignitors, direct spark ignitors, and single-rod flame rectification sensors on residential and light-commercial gas furnaces and gas package units. Does not cover standing pilot and thermocouple systems, owned by the thermocouple and standing pilot service SOP; the no-heat triage that dispatched the call, owned by the no-heat emergency call response SOP; or the combustion tuning that follows a gas train change, owned by the gas furnace combustion analysis and tuning SOP.

Roles and handoffs

Procedure

1. Read the board's fault code and watch one full trial for ignition before removing anything. Record the code as flashed, then observe inducer, pressure switch, ignitor warmup, valve open, flame proof, blower. Acceptance: a written sequence naming where it stopped. Wrong looks like cycling power first, which erases the stored code. Stop rule: if it never reaches ignitor warmup the fault is upstream and this SOP does not apply yet. Hazard: this fires the burner on a suspected combustion fault, so the personal CO monitor goes on your body before the call for heat and stays on for the whole visit; stand clear of the burner face and never defeat the door switch while the burner lights.

2. Close the appliance gas cock, kill power at the disconnect, and prove the control compartment dead. Acceptance: meter proved on a known live source, zero at the line terminals, proved live again after. Wrong looks like trusting the switch on the furnace jamb, which on many installs is downstream of nothing useful. Stop rule: if you cannot identify the disconnect serving the appliance, trace it before any panel comes off. Hazard: this puts a hand into the control compartment, and 1910.333(b)(2) practices apply to a qualified person under 1910.332.

3. Measure what the board actually delivers to the ignitor during trial, before condemning the ignitor. Back-probe the leads, close the doors, call for heat, read during warmup. Acceptance: the reading matches the family stamped on the part, nominal 120 V or nominal 80 V, within the tolerance the furnace literature states. Wrong looks like a 120 V ignitor on an 80 V board or the reverse, the most common reason a new ignitor lasts one season. Stop rule: wrong family voltage means the board is the repair. Hazard: read energized on a firing burner, so the personal CO monitor stays in your breathing zone, use leads rated for the circuit, and keep a probe from bridging to the cabinet.

4. With power off, measure the removed ignitor cold, inspect the ceramic under light, and name why it died. Acceptance: cold resistance inside the range the replacement part's data sheet publishes for that family, no fracture, no white bloom, no oil film from a bare hand, plus one named cause: wrong-family voltage, thermal shock from short cycling on another fault, contact with the burner or shield, or handling contamination. Wrong looks like "ignitor failed" as both symptom and cause. Stop rule: an ignitor in range and clean is not the fault, so move the meter to the flame side; a second ignitor on one unit inside twelve months with no named cause goes to the service manager. Hazard: fragments and bracket edges cut through a thin glove, so cut-resistant gloves and eye protection for the removal.

5. Fit the new ignitor by its bracket, handling only the ceramic base. Acceptance: element at the manufacturer's stated position relative to the burner port, gasket intact, no metal contact, cold resistance re-measured in place and inside the data sheet range. Wrong looks like a fingerprint on the element, or an element pushed off position to make the bracket line up. Stop rule: if the bracket will not seat without bending the element toward the burner, you have the wrong part. Hazard: hands are in the burner compartment, so confirm the gas cock is closed and the ignitor circuit dead before reaching in.

6. Restore gas, then measure flame signal in microamps before cleaning the sensor and again after. Reopen the appliance shutoff with the burner compartment closed, confirm no odor at the cabinet before calling for heat, and leak-check any joint disturbed. Break the sensor lead, meter in series in DC microamps, read at steady flame. Acceptance: a steady reading at or above the minimum the furnace literature states, with no downward drift over a two minute burn. Wrong looks like a reading that starts acceptable and decays, which is a ground path or a rich burner rather than a dirty rod. Stop rule: if a non-abrasive pad does not lift the signal above the stated minimum, stop polishing and check the burner ground path. Hazard: read with the burner firing and leads in a live cabinet, so route leads clear of the blower wheel and keep the personal CO monitor in your breathing zone.

7. Prove the flame safety still shuts the appliance down. With gas and power on from step 6, run one full call to steady flame with the doors on, then close the manual gas cock with the burner firing and watch the lockout. Acceptance: burners drop out, the board runs its retries and locks out as its literature describes, valve closed. Wrong looks like the board holding the valve open with no flame present. Stop rule: a furnace that will not lock out on loss of flame is shut off and tagged, and the board is the repair. Hazard: this is where fuel and energy go back in with the family in the house, so stand to the side of the burner face and confirm the door switch kills the unit when the door is pulled, because the diagnostic magnet has to come off before you leave.

8. Take post-repair combustion readings and write the file. Acceptance: flue carbon monoxide air-free against the 400 ppm air-free limit the ANSI Z21 series sets for listed vented appliances, in the edition the appliance was listed to, which binds the manufacturer and reaches you through the appliance's own literature - it is a listing-test limit, so treat it as a ceiling and not a target - or the manufacturer's lower limit where stated; plus ambient CO in the space with the appliance firing, at or below your monitor's alarm setpoint. Wrong looks like closing a combustion repair with no combustion number on it. Stop rule: air-free over the limit means the appliance stays off pending the tuning SOP, and any ambient reading above your alarm setpoint stops the visit: appliance off, space ventilated, nothing else attempted. Hazard: you are beside a firing appliance, so the personal monitor stays on until you are out of the space.

The record this produces

One repair record per visit, attached to the equipment:

• Fault code as flashed, and the observed sequence with its stopping point
• Ignitor circuit voltage during warmup, the family stamped on the part, and cold resistance of the removed and the new ignitor against the data sheet range
• Flame signal in microamps before cleaning and after, and the stated minimum for that furnace
• The named cause of the old part's failure, with its evidence
• Lockout behavior on the deliberate flame-loss test, and door switch confirmed working
• Post-repair flue CO air-free with its limit, and ambient CO with the appliance firing

The next tech reads the flame signal history to know whether a marginal rod is drifting. The manager reads named causes across the book: three ignitors on one model in a season is a board or a venting pattern.

Worked pass: 9-year-old 80 percent furnace, ignition lockout code

Step 1: board flashes an ignition-failure code. Sequence runs inducer, pressure switch closes, ignitor warms, valve opens, no flame proof, three retries, lockout. Stopping point written as flame proof.

Step 2: gas cock closed at 09:12, power off at the attic disconnect, line terminals proved dead with the meter checked live before and after.

Step 3: ignitor circuit reads 118 V during warmup against the 120 V family stamped on the bracket, so the board is delivering the correct family voltage.

Step 4: the removed ignitor measures 61 ohms cold against the replacement part's published range of 40 to 90 ohms for that 120 V silicon carbide family, read off the data sheet in the box. In range, and under light the element is intact with a light gray bloom on one leg. Named cause: none, because the ignitor did not fail. Step 6 FAILS on the first pass and takes its stop rule. Gas reopened with the compartment closed and no odor at the cabinet, and the burner lights then drops out at about four seconds. Flame signal reads 0.4 microamps at steady flame against the 1.0 microamp minimum this furnace's literature states, 0.6 microamps short. The rod is cleaned with a non-abrasive pad and re-measured at 0.5 microamps, still under the minimum, so the stop rule is taken rather than polishing again. The burner ground screw is found backed out two turns with its star washer sitting on paint. Screw cleaned to bare metal and torqued down, continuity confirmed to the cabinet. Flame signal re-measured at 4.1 microamps, steady over a two minute burn with no downward drift, which clears step 6's acceptance as written.

Step 7: one full call to steady flame with doors on. Manual gas cock then closed with the burner firing: burners drop out, board runs three retries and locks out with the valve closed. Door switch tested by pulling the door, unit drops out, diagnostic magnet confirmed off.

Step 8: flue CO reads 38 ppm air-free against the 400 ppm air-free ANSI Z21 series limit, with no lower manufacturer limit stated on this model. Ambient in the utility room reads 0 ppm with the appliance firing, under this shop's monitor alarm setpoint.

Self-audit: the 0.4, 0.5 and 4.1 microamp readings are the same rod at the same series point against the same stated 1.0 minimum, and 0.5 is reported as a failure rather than a partial save. Step 7's flame-loss test used the manual gas cock, so no fault was induced on a live gas control.

References

• ANSI Z21 series listing limits for carbon monoxide in the flue products of listed vented appliances, at 400 ppm air-free, in the edition the appliance was listed to, superseded by a lower manufacturer limit where one is stated
• The furnace's own service literature for the minimum flame signal and the ignitor family voltage, and the replacement ignitor's data sheet for its published cold resistance range
• 29 CFR 1910.333(b)(2) with NFPA 70E-2021, 120.5, for proving dead before opening a control compartment; 29 CFR 1910.134 for respiratory protection under a written program
• See related: the no-heat emergency call response SOP and the gas furnace combustion analysis and tuning SOP