Furnace Ignition Failure Troubleshooting
A furnace that won't light isn't a mystery — it's a sequence that stopped somewhere. Walk the ignition steps in order, from the call for heat to flame proving, and the failed part shows itself.
Stuck on a no-heat call?
Ventora's guided troubleshooting walks the ignition sequence with you and points to the likely part.
In This Guide
How Modern Furnace Ignition Works
Almost every furnace built in the last three decades uses an integrated furnace control (IFC) board that runs a fixed, safety-interlocked sequence. Nothing skips ahead. Each step has to prove itself before the board allows the next one. Understand that order and diagnosis becomes a matter of finding where the chain broke rather than guessing at parts.
On a typical hot surface ignition (HSI) furnace the sequence is:
- Call for heat — the thermostat sends 24V to W at the board.
- Self-check — the board confirms the high-limit is closed and the pressure switch is open (proving it isn't stuck closed).
- Pre-purge — the inducer (draft) motor energizes and pulls a negative pressure that closes the pressure switch, proving the flue is clear.
- Igniter warm-up — the HSI is energized and glows to roughly 1,800–2,500°F over about 15–45 seconds.
- Trial for ignition — the gas valve opens and the burners light.
- Flame proving — the flame sensor rectifies the flame into a small DC microamp signal. If the board sees flame within the trial period (usually about 4–7 seconds), it de-energizes the igniter and keeps the valve open.
- Blower on-delay — after roughly 30–45 seconds the circulating blower starts on heat speed.
If flame isn't proven, the board closes the gas valve and retries — typically three attempts — before going into a one-hour hard lockout. Spark-ignition and standing-pilot systems follow the same logic; only the ignition source and the flame-proving device change.
Start With the Blink Code
Before you pull a single wire, read the diagnostic LED through the sight glass. The board stores the last several faults, and that history tells you where in the sequence the furnace stopped. Chasing a part without reading the code is how techs replace good igniters.
Reading the LED
Codes are usually shown as slow/fast flashes or a two-digit blink pattern (e.g., 3-3 for an open pressure switch, 3-4 for a limit circuit fault). The legend is printed on the blower-door label — always read the code against that specific manufacturer's chart, because a "4-flash" means different things across brands.
Common stored faults and what they point to: lockout / failed ignition (no flame proven), flame sensed with no call (leaky or slow-closing gas valve), pressure switch open (draft problem), and limit open (airflow problem). Note the code, then verify it — the code tells you where to look, not what to replace.
Walking the Sequence Step by Step
Put your meter on the board and follow the same order the furnace does. Each step either passes or hands you the fault.
Step 1 — Prove the call. Measure 24V between W and C at the board. No 24V and the fault is the thermostat, wiring, or 24V transformer, not the furnace itself.
Step 2 — Watch the inducer. It should spin up immediately. If it's dead, check for line voltage to the motor and inspect the run capacitor. A weak inducer that spins slowly may never pull enough vacuum to close the pressure switch.
Step 3 — Prove draft. Confirm the pressure switch closes once the inducer is up. Put a manometer on the switch port; a residential single-stage switch typically needs roughly 0.40–1.0 in WC of negative pressure to make.
Step 4 — Watch the igniter. On an HSI system it should glow bright orange. No glow means the igniter, its wiring, or the board's igniter output.
Step 5 — Prove gas. Look for 24V at the valve during the trial and confirm the burners light. If you have 24V and no flame, the valve or the gas supply is the issue.
Step 6 — Prove flame. If the burners light and then die a few seconds later, the flame sensor never told the board it saw fire. That's a microamp problem, covered below.
Safety first
You're working around natural gas or propane and 120V. Smell for gas before you cycle the unit, and never jumper out the pressure switch or high-limit to "test" — those interlocks protect against a cracked heat exchanger and blocked flue. If a switch keeps tripping, find out why.
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Testing the Hot Surface Igniter
If the igniter never glows, isolate it fast. Kill power, disconnect the two-pin igniter plug, and measure resistance across the element. The reading depends on the technology:
| Igniter type | Typical cold resistance | Notes |
|---|---|---|
| Silicon carbide (legacy 120V) | 40–90 Ω | Brittle; never touch the element with bare fingers |
| Silicon nitride (universal) | 40–200+ Ω | More durable; match the exact replacement kit |
| Open element | OL / infinite | Cracked or burned out — replace |
An open reading (OL) means a cracked or failed element — replace it. A reading in range but no glow points to the board's igniter output or the harness. Confirm by checking for line voltage at the igniter plug during warm-up; if voltage is present and the element is good, it will glow.
Don't touch the element
Oil from your skin creates a hot spot on the silicon element that leads to early failure. Handle a new igniter by its ceramic base and porcelain only, and check for hairline cracks before you install it.
Flame Sensing and Rectification
A dirty flame sensor is the single most common furnace no-heat call. When the burners light, drop out after a few seconds, and the furnace retries, the flame sensor is the prime suspect. Here's the physics: a flame conducts current, and because the sensor rod is much smaller than the grounded burner, the flame acts as a one-way valve — it rectifies AC into a small DC current the board can read. The board is literally proving a flame exists by measuring that microamp signal.
Measuring flame current
Put a meter in series with the flame-sensor wire on the microamp (µA DC) scale, or use a meter with a dedicated flame-current setting. Read while the burners are lit.
| Flame current | Meaning |
|---|---|
| 4–6 µA | Healthy — clean rod, good ground, solid flame |
| 1.5–3 µA | Marginal — clean the rod, verify ground and flame impingement |
| < 1 µA | Below dropout — board loses flame and locks out |
| 0 µA | No signal — dirty/cracked rod, bad ground, or flame not touching rod |
Most boards drop out around 0.5–1.0 µA. If you read low, pull the sensor and clean the rod with a fine abrasive pad, ScotchBrite, or steel wool — never sandpaper, which leaves a grit residue that insulates the rod. Reinstall and re-read. If cleaning brings it back to 4–6 µA, you're done. If it stays low with a clean rod, check the burner ground and the flame position; a sensor that isn't fully in the flame reads weak no matter how clean it is.
Ground matters
Flame rectification depends on a solid burner-to-chassis ground. A corroded burner mount or a loose ground screw shows up as a low or erratic microamp reading even with a spotless sensor. Verify the ground path before you condemn the sensor.
Gas Valve and Pressure Switch
If the igniter glows but the burners never light, the problem is gas delivery or draft proving. Split it into two checks.
Pressure switch (draft). On 90%+ condensing furnaces a blocked condensate trap, plugged drain, or restricted flue is a leading cause of no-ignition. The inducer runs but can't pull enough vacuum, so the switch never closes and the board holds before the igniter. Measure vacuum at the switch and inspect the trap, tubing, and terminations. Never jumper the switch closed.
Gas valve. Confirm the valve gets 24V during the trial for ignition. If it has power and the burners don't light, either the valve isn't opening or there's no gas. Check the manual shutoff, then verify manifold pressure with a manometer once lit:
- Natural gas manifold: typically 3.5 in WC on a single-stage valve
- Propane (LP) manifold: typically 10–11 in WC
- Inlet supply: roughly 5–7 in WC for NG, 11–14 in WC for LP
Always set manifold pressure to the rating plate for that specific furnace and gas type. If the valve has 24V but won't open, and the supply and manual valve check out, the gas valve is faulty.
Worked Example: Ignites Then Drops Out
The call:
Homeowner reports intermittent no-heat. On arrival the furnace lights, runs 5–6 seconds, shuts off, and retries. After three tries the LED shows a lockout code.
Step 1: Read the code — stored fault is "failed ignition / flame not sensed." Igniter glows and burners light, so the failure is downstream at flame proving.
Step 2: Meter in series on the flame-sensor lead, µA DC. Reading while lit: 0.7 µA — below the board's dropout threshold.
Step 3: Pull the sensor. Rod is coated with a dull scale. Clean with a ScotchBrite pad, reinstall.
Step 4: Re-read flame current: 5.2 µA. Cycle the furnace three times — lights and holds every time.
Result: Resolved by cleaning the sensor
Flame current went from 0.7 µA (below dropout) to 5.2 µA (healthy). The sequence walked us straight to the cheapest fix — no parts, just a clean rod. Note it on the invoice and recommend a maintenance visit so it doesn't recur mid-winter.
If cleaning hadn't recovered the signal, the next moves would be verifying the burner ground and confirming the flame fully engulfs the rod — then replacing the sensor as a last step.
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