Furnace Igniter Glows No Gas Decision Tree

Why this matters

A furnace where the hot surface igniter glows but the gas valve never opens is reporting one of four conditions: pressure switch open during ignition trial, gas valve electrical fail, gas supply cut off, or control board ignition logic locked out. Each has a distinct test signature and the system tells you which one with a meter and a manometer in 5 minutes. Sell the wrong part and you've still got a no-heat customer.

Symptom presentation

Sequence on a call for heat:

• Inducer fan starts.
• Pressure switch closes (audible click).
• Igniter glows orange-white.
• Gas valve does not open: no whoosh of ignition, no flame, no whoof of failed ignition.
• Igniter eventually cools after the trial period.
• Board attempts 2 or 3 more trials, then goes to hard lockout.

Some boards display a blink code matching "no flame established" or "ignition failure." Read the code before tearing in.

Quick checks (under 2 min)

Step 1: Confirm gas is on at the appliance shutoff (the valve on the gas line right before the furnace). Many calls end here - a remodeler shut it off, never turned it back on.

Step 2: Confirm gas is on at the meter. Look at another gas appliance (water heater, range) and verify operation.

Step 3: Manometer to the gas valve outlet tap. With the system trying to fire, watch for manifold pressure to spike. If the valve is opening electrically but no gas is in the line, manifold reads zero. If the valve is not opening at all, manifold also reads zero.

Step 4: Voltage check at the gas valve terminals during the ignition trial. 24 VAC present = board is calling, valve isn't opening. 0 VAC present = board isn't calling, problem is upstream.

Isolation tree

Branch 1: Gas supply cut off.

• Appliance shutoff valve closed: open it. Confirm direction (handle inline with pipe = open, perpendicular = closed).
• Meter shutoff closed: check at the gas meter; call utility if customer can't restore.
• LP tank empty: gauge reads zero; refill before re-test.
• New construction or recent line work: line may be air-locked. Bleed at a downstream appliance.

Branch 2: 24 VAC present at gas valve, gas supply confirmed, but valve does not open. Gas valve electrical or mechanical fail. Diagnosis:

• Read coil resistance across the valve terminals: open winding = replace valve.
• Coil reads OK but valve doesn't open: solenoid stuck mechanical, valve still gets replaced.
• Some two-stage valves have a separate first-stage and second-stage coil; check both.

Branch 3: 0 VAC at gas valve during ignition trial. The board is not energizing the valve. Look upstream:

• Pressure switch opened during ignition trial (not staying closed under trial load). Read across the pressure switch terminals during trial: should remain closed (under 0.5 VDC drop). If it opens, the inducer flow is marginal - check vent obstruction, condensate trap blockage in condensing furnaces, weak inducer.
• Roll-out switch tripped (manual reset, opens on high temperature in the burner area; indicates a flame roll-out event from a prior cycle).
• High-limit switch tripped (typically manual reset on furnaces with stuck blower; indicates an overheat event).
• Flame roll-out reset switch: physical button on the burner box; reset, watch for repeat.
• Door switch on the blower compartment: open switch = no operation. Verify door is fully closed.

Branch 4: Pressure switch nuisance fault. The switch reads closed at static (inducer running), but the small additional load of opening the gas valve creates enough flue draft change to drop the switch open momentarily. Common on condensing furnaces with marginal venting.

• Inspect vent termination outdoors for snow, ice, leaves, animal nesting.
• Inspect condensate trap and drain - a partial clog raises flue back pressure.
• Inspect vent slope and length against OEM limits in the install manual.

Branch 5: Flame rectification check before condemning the valve. Modern boards self-check the flame sensor signal before opening the valve on later trials. A grounded or shorted flame sensor can prevent the board from opening the valve.

• Disconnect the flame sensor lead, retest. If valve now opens (and the system runs briefly), the flame sensor or its lead was the cause.

Branch 6: Board ignition logic locked. After multiple trial-for-ignition attempts, modern boards hard-lock until they get a power-cycle. Customer-side power cycle (thermostat off for 5 minutes, breaker off for 30 seconds) may reset and let you observe the actual failure mode rather than a downstream lockout symptom.

Branch 7: LP-only complications. On LP systems, the OEM may require an LP conversion kit with different orifices and a different manifold pressure setting (NG ~3.5 in. w.c. vs LP ~10 to 11 in. w.c.). A furnace converted at install but reverted to NG settings during a control board swap will fail to ignite or run dangerously rich. Confirm conversion status on the data plate and inspect the orifices.

Confirming the diagnosis

After repair, run three consecutive cycles:

• Ignition completes within OEM trial time (typically 4 to 7 seconds).
• Manifold pressure within OEM spec at static and running.
• Flame characteristic blue, fully enveloping the sensor.
• Combustion analyzer reads:
  • CO under 100 ppm air-free (target under 50 ppm on most modern furnaces).
  • O2 in expected range for the burner type.
  • Stack temperature consistent with OEM spec.

Document on the invoice. Combustion-analyzer data is the gold standard for proving you didn't leave a safety problem behind.

Repair / remediation

• Gas valve replacement: OEM part number, match physical inlet/outlet sizes, voltage, regulator type (NG vs LP).
• Pressure switch: match make-pressure and break-pressure rating (typically printed on switch).
• Vent or condensate trap: clear obstruction, verify slope, replace any compromised components.
• Flame sensor / wire: cheap and fast, do it preemptively if remotely suspect.
• Control board: last resort, OEM part, document full diagnostic before swap.

References

• NFPA 54 / ANSI Z223.1 - National Fuel Gas Code
• International Fuel Gas Code (IFGC) Chapters 4 through 6
• OEM service manuals (Carrier, Trane, Lennox, Goodman, Rheem - ignition sequence and pressure switch specs)
• ASHRAE Handbook - HVAC Systems and Equipment