What you'll see

Thermostat is calling for heat, but no flame. Maybe nothing happens at all, maybe the inducer spins and then it quits, maybe the igniter glows but no gas lights. A furnace ignites in a fixed sequence, and a no-ignition call is just "which step did it die on." Watch one full attempt start to finish and note where it stalls — that one observation does most of the diagnosis for you.

If the board has a diagnostic LED, read the flash code first. It usually points you straight at the stalled step.

Walk it in order — follow the sequence of operation

  1. Confirm the call and 24V. Stat set to heat, setpoint above room temp. Verify 24V on W at the board. No call, no ignition — start at the thermostat.
  1. Inducer (draft) motor. On a call, the inducer should spin up first to prove draft. If it doesn't run: check power to it, its capacitor (if equipped), and the board output. If it runs but the sequence still stalls right after, suspect the pressure switch isn't closing.
  1. Pressure switch. Once the inducer establishes draft, the pressure switch should close and prove it. Measure across it. Won't close → blocked flue/intake, cracked or disconnected pressure hose, water in the hose (high-eff), a weak inducer, or a clogged condensate drain backing up on a condensing furnace. Don't jumper it shut to "test" — it's proving you won't dump combustion products into the house.
  1. Igniter / ignition source. With draft proven, the igniter energizes. On a hot-surface igniter you should see it glow bright orange. No glow → check for the ~120V (or line) signal to the igniter and the igniter's resistance; a cracked HSI reads open or doesn't glow. On spark ignition, look/listen for a steady spark. A common, simple fix here is a cracked igniter.
  1. Gas valve opens. After the igniter is hot (or sparking), the board energizes the gas valve. Confirm 24V at the valve at the right moment. Got 24V but no gas → valve not opening, or no gas supply (closed manual valve, tripped meter, LP tank empty, last guy left the valve off). No 24V to the valve → board or a safety in the chain held it off.
  1. Flame proves and stays. Gas lights, the flame sensor proves it, and the main flame stays lit. If it lights and then drops out a second later, you've crossed into a flame-sense problem — that's its own call (see the flame-sense lockout article).
  1. Limit switch. A high-limit open (often from a prior overheat with bad airflow) can block the sequence. Check the limit for continuity; if it's open with a cold furnace, it's failed or there's a residual airflow problem.

What "normal" looks like

  • 24V on W: present through the whole heat call.
  • Inducer: spins up promptly and runs the entire cycle.
  • Pressure switch: closes shortly after the inducer proves draft; stays closed.
  • HSI: glows bright orange; cold resistance is typically tens of ohms (silicon nitride differs from silicon carbide — go by the part).
  • Gas valve: 24V applied at the ignition step; manifold pressure comes up to spec once open (commonly ~3.5″ W.C. for natural gas, ~10–11″ W.C. for LP — go by the rating plate).
  • Lockout: most boards retry a few times, then lock out and flash a code.

Common faults & what they mean

  • Cracked hot-surface igniter: no glow, no light. Reads open or visibly cracked. Extremely common; handle the new one without touching the element.
  • Pressure switch won't close: sequence stalls after the inducer. Blocked vent/intake, bad hose, water in the hose, weak inducer, or a condensate backup.
  • No gas supply: igniter glows, valve gets 24V, nothing lights. Manual valve off, empty LP, meter issue, or air in the line after work was done.
  • Failed gas valve: 24V present at the valve but it won't open. Confirm signal timing before condemning it.
  • Open high limit: blocks ignition with a cold furnace if failed; if it opens during run, you have an airflow/overheat problem.
  • Board / flame-sense lockout: tries and locks out. Read the flash code; a lockout after a brief flame is flame-sense, not no-ignition.

Tech tips & gotchas

  • Read the flash code before you touch anything. It often names the stalled step (pressure switch open, ignition lockout, limit open) and saves you a guessing tour.
  • Watch a full cycle with your eyes on the burner. The exact moment it fails — before the inducer, after draft, no glow, glow-but-no-gas, light-then-drop — is the diagnosis.
  • Never jumper a pressure switch or a limit to make it run. Those prove safe combustion and safe temperature. Bypassing them can vent CO into the house.
  • Don't touch a new HSI's element with bare fingers. Skin oils create hot spots that crack it early.
  • Check the obvious gas supply. More "no ignition" calls than you'd think are a manual valve someone left off or an empty propane tank.
  • Confirm the condensate drain on condensing furnaces. A backed-up drain trips the pressure switch and reads as a draft fault.

Safety / code notes

  • Any no-heat call is also a combustion-safety check: confirm proper venting and intake are clear and intact, and that nothing's been bypassed.
  • Verify gas piping and appliance connections are sound; size and material per IFGC/IMC fuel-gas provisions — cite the section, never improvise the pipe.
  • If you smell gas, find and stop the leak before energizing anything. Use a combustion analyzer to verify safe operation after a repair, and check for CO.
  • Combustion-air openings must meet IMC §701 sizing for the space — a starved burner is a safety issue, not just a performance one.