What it is

These are the two small parts that light a modern gas furnace and prove it stayed lit:

  • Flame sensor (flame rod) — a single metal rod sitting in the burner flame. It does NOT make heat or spark; its only job is to confirm a flame is present, using a trick called flame rectification. If the board doesn't sense flame within a few seconds of opening the gas valve, it shuts the gas off. This is the part that gets dirty and causes the most no-heat callbacks.
  • Hot surface igniter (HSI) — a small ceramic element that glows orange-hot when line voltage runs through it, lighting the burners. It replaced standing pilots and spark on most furnaces. Two materials are common — older silicon carbide (fragile, lower resistance) and newer silicon nitride (tougher, higher resistance).

This article covers identifying each, the values that tell you if they're good, and how they fail. (For the full ignition sequence, see the ignition-systems topic; for flame-sensing theory in depth, see that topic.)

How it works

Flame rectification (the flame sensor): The board puts an AC voltage on the flame rod. A flame is electrically conductive, but it conducts much better in one direction than the other (because the burner ground area is far larger than the small rod tip). So the flame acts like a diode and rectifies the AC into a tiny DC current — microamps — that flows from the flame to ground. The board measures that DC microamp signal. Real flame = steady DC microamps = "flame proven." No flame, or a dirty/cracked rod = too few microamps = the board shuts the gas. The flame literally completes the circuit; the rod just samples it.

The hot surface igniter: Line voltage through the ceramic element heats it to incandescence (well past ignition temperature). The board energizes it during a warm-up window, then opens the gas valve so the gas lights off the glowing element. Once flame is proven, the igniter de-energizes. Silicon carbide igniters are brittle and run at a lower resistance; silicon nitride igniters are more robust, often run at higher resistance, and tolerate more on/off cycles.

In the field

Test the flame sensor by microamps. Put a meter set to DC microamps in series with the flame-sense wire (break the connection, meter in line) and run the burner. Read the steady microamp value with flame established. A healthy signal is a solid, stable reading; a weak or jumpy reading means a dirty or failing rod, a bad ground, or a cracked porcelain insulator.

Clean a weak flame rod — gently. Light scuffing with fine abrasive (a non-aggressive pad or fine steel wool) removes the oxide/silica film that blocks the signal. Don't sandblast it or use coarse paper — you'll remove material and shorten its life. Re-read microamps after cleaning; a good clean often jumps the reading way up.

Check the ground. Flame rectification needs a solid burner ground and correct line polarity. A bad ground or reversed hot/neutral starves the microamp signal even with a clean rod — the furnace lights then drops out. Verify ground and polarity before condemning the sensor.

Test the HSI by resistance (cold, power off). Ohm across the igniter leads. Silicon carbide igniters read low resistance (often roughly 40–90 ohms cold); silicon nitride igniters read higher (often well over 100 ohms, into the several-hundred-ohm range depending on the part). An OPEN reading (OL) means a cracked/broken element — no heat, no ignition. Compare to the part's spec.

Inspect the HSI visually. A hairline crack in the glowing element kills it. Look for cracks, white-hot vs dull spots, and physical damage. Don't touch the element with bare fingers on silicon carbide types — skin oil can create a hot spot and shorten its life (silicon nitride is more forgiving).

Normal values & targets

  • Flame-sense microamps (healthy): a steady DC current, commonly in the range of about 2–10 µA on residential furnaces (some run a few µA, some higher). The exact "good" number is per the board, but stable and well above the board's minimum dropout (often around 0.5–1 µA) is what you want.
  • Flame-sense dropout threshold: boards typically need roughly 0.5–1.5 µA minimum to call flame proven; below that, it shuts down. A reading hovering near the threshold = imminent nuisance lockouts.
  • Silicon carbide HSI resistance (cold): commonly about 40–90 ohms.
  • Silicon nitride HSI resistance (cold): commonly higher, often 100+ ohms (varies widely by part — check spec).
  • HSI supply: line voltage (120V) during the warm-up window; many newer silicon-nitride igniters run on lower voltage/current than old carbide types.
  • Igniter open (OL): cracked/failed element — no continuity, no heat.

Common faults & what they mean

  • Furnace lights, runs ~5–10 seconds, then shuts off and may retry/lock out → classic weak flame signal: dirty flame rod, bad ground, cracked rod insulator, or reversed polarity. Read microamps.
  • Microamps low even after cleaning → bad burner ground, reversed line polarity, cracked porcelain on the rod, or a failing board flame-sense circuit. Don't just keep cleaning.
  • Igniter doesn't glow, reads OL → cracked/open igniter element; replace.
  • Igniter glows but burners don't light → igniter is fine; look at gas (valve, manifold pressure, supply) or igniter position relative to the burner.
  • Igniter glows then cracks/fails repeatedly → overvoltage, vibration, contamination (handling a carbide igniter), or an igniter mismatched to the furnace.
  • Flame signal drops as the furnace warms up → expanding metal shifting the rod out of the flame, or a marginal connection that opens when hot.

Tech tips & gotchas

A dirty flame sensor is the single most common no-heat callback. It's a five-minute clean, not a part swap — pull it, lightly scuff the rod, reinstall, re-read microamps. Many "intermittent furnace" complaints are exactly this.

But don't reflexively clean-and-go. If microamps are still low after cleaning, the problem is ground, polarity, or a cracked insulator — not dirt. Chasing it with repeated cleanings wastes trips.

Reversed line polarity and a missing/poor ground both kill flame rectification because the signal has to return to ground through the burner. Check polarity and ground early; they're invisible if you only look at the rod.

For igniters, know your material. A silicon carbide igniter is fragile — handle it by the base, never the element, and never touch the element. Silicon nitride is tougher and often the better replacement, but match the resistance/voltage spec to the furnace so the warm-up timing stays right.

A cracked igniter element can look intact — the crack may only show when it's energized (one section stays dull). Ohm it cold; an open reading confirms it even when it looks fine.

Position matters. If you replace an igniter or flame rod, set it back to the original depth/angle in the flame path. A rod that's out of the flame reads no signal; an igniter out of position won't light the burner reliably.

Safety / code notes

Flame sensing is a primary safety function — it's what shuts the gas off when the burner fails to light or goes out. Never bypass or jumper the flame-proving circuit; doing so can dump unburned gas. Verify correct line polarity and a solid equipment ground, which the safety/sensing circuit depends on. Match the replacement igniter to the furnace (material, resistance, voltage) so ignition timing and the listed safety sequence stay correct. After service, confirm a stable flame signal and proper combustion before leaving.