What you'll see

The burner lights — you see real flame — and then a few seconds later it cuts off. The furnace may retry a couple of times and then lock out and flash a code. This pattern is the signature of a flame-proving failure: the board lit the gas but never "saw" the flame through the flame sensor, so it shut the gas off as a safety. It's one of the most common no-heat calls, and it's usually a cheap fix — but techs throw boards and gas valves at it because they don't recognize the pattern.

The flame is there. The problem is the furnace can't confirm the flame.

How the proving works (briefly)

The flame sensor is a single metal rod in the flame. The board pushes a small AC signal through it; flame conducts current in essentially one direction (rectification), so a tiny DC microamp current flows back to the board through the flame and ground. If that microamp signal is too weak, the board assumes there's no flame and drops the gas valve. Anything that weakens or breaks that current path causes the light-then-die symptom.

Walk it in order

  1. Confirm the pattern. Watch a cycle: igniter heats, gas lights, flame is visibly present, then it drops out after roughly 1–5 seconds. Read the flash code — "flame failure" or "ignition lockout after flame" confirms flame sense. (If it never lights at all, that's the no-ignition tree instead.)
  1. Measure flame current in microamps. Put a meter in series with the flame-sense lead (microamp DC). A healthy reading is typically a few microamps and above — often 2–6 µA, with the board's dropout threshold lower than that. A reading near zero with visible flame = the proving path is broken or starved.
  1. Pull and inspect the flame rod. A film of oxidation, soot, or white scale insulates the rod and kills the signal. Clean it gently with fine abrasive (steel wool or light sandpaper) — not aggressive grinding. Reinstall and re-measure microamps. This alone fixes a huge share of these calls.
  1. Check the rod's position in the flame. It has to sit in the flame to conduct. A bent rod, a rod that's drifted out of the flame, or a burner that's lit unevenly (dirty burners, wrong manifold pressure) starves the signal. Confirm the flame is impinging on the rod.
  1. Check ground. Flame rectification needs a solid ground reference — the burner assembly to the board to the furnace chassis. A corroded burner ground, a loose chassis ground, or a bad board ground tanks the microamps. Verify the ground path.
  1. Check line polarity and neutral. Flame sensing is polarity-sensitive. If hot and neutral are swapped at the furnace, or the neutral/ground is poor, the board can't rectify properly and you'll get a phantom flame-failure. Confirm correct polarity at the furnace.
  1. Only then suspect the board or valve. If the rod is clean and well-placed, ground and polarity are good, the flame is strong, and microamps are still near zero — now the sensing circuit on the board is suspect. The board is the last stop, not the first.

What "normal" looks like

  • Flame current: commonly a few microamps to around 6–10 µA on a clean, well-positioned rod. Trend matters: a reading that sags toward the board's dropout threshold over a few seconds is a dying signal.
  • Trial-for-ignition: the flame must prove within the board's flame-establishing period (a few seconds) or the gas valve closes.
  • Retries/lockout: typically a few attempts, then lockout with a flash code; many boards auto-reset after an hour.
  • Polarity: hot and neutral correct at the furnace; solid equipment ground.

Common faults & what they mean

  • Dirty/oxidized flame rod: visible flame, microamps near zero, cleans up and reads fine. The most common cause by far.
  • Rod out of the flame / weak flame: dirty burners, low manifold pressure, or a bent rod means the flame isn't bathing the rod. Fix the flame, not just the rod.
  • Bad ground: corroded burner-to-chassis or board ground starves rectification. Microamps stay low even with a clean rod.
  • Reversed polarity / poor neutral: board can't rectify; intermittent or total flame-proof failure. Check the receptacle/wiring.
  • Cracked porcelain on the rod insulator: shorts the signal to ground. Replace the sensor.
  • Failed sensing circuit on the board: everything else checks out and microamps are still dead. Last resort.

Tech tips & gotchas

  • Clean the rod before condemning anything. It's a five-minute fix that solves most of these calls. Don't sell a board until the rod is clean and measured.
  • Measure microamps — don't eyeball it. "It looks lit" is exactly the problem; the board needs current, not your opinion. The meter turns a guess into a diagnosis.
  • A sagging microamp reading predicts a future no-heat. If it lights and holds at a marginal value, it'll drop out on a cold night. Clean it now even if it's "working."
  • Don't sand the rod down to nothing. Light cleaning only; aggressive abrasion thins and shortens the rod's life.
  • Reversed polarity hides as a board failure. Before you sell a board, confirm hot/neutral and ground — a swapped receptacle has fooled plenty of techs.
  • Weak flame is a sensing problem too. If the burners are filthy or the gas pressure is low, the flame won't conduct well no matter how clean the rod is.

Safety / code notes

  • The flame-proving system is a safety: it shuts gas off when it can't confirm combustion. Never bypass it or hold the valve open manually to "keep heat on."
  • Correct line polarity and a solid equipment ground are required for safe, reliable operation — fix wiring faults, don't work around them.
  • After any burner or gas-side work, verify combustion and check for CO with an analyzer; confirm venting and combustion air are intact per IMC §701.