Furnace Igniter Glows But Burner Won't Light: Gas Valve, Flame Sensor, or Pressure?

If your furnace igniter glows orange but the burner never fires, the igniter itself isn’t the problem. The ignition sequence kept going past that step and stalled somewhere else. The three most common culprits are the pressure switch, flame sensor, and gas valve. A technician can tell which one in about 15-20 minutes.

Why the igniter can glow without a burner firing

Modern furnaces run an ignition sequence controlled by the board. The igniter heats up first, then the board sends a signal to open the gas valve, then the burner lights, then the flame sensor confirms the flame is there and tells the board to keep the valve open. If anything in that chain fails, the board shuts things down and you get no heat, even though you saw the igniter do its job.

The igniter glowing is useful information: it tells you the board is alive, the igniter circuit is intact, and the sequence started. The failure is somewhere in the middle or end of that chain.

The pressure switch (most common culprit)

Before the gas valve ever opens, the board checks that the inducer motor is running and pulling enough draft. It does this through one or more pressure switches, small round discs with a rubber hose connected to a port on the inducer housing. If the pressure switch doesn’t close, the board won’t send voltage to the gas valve, period.

A clogged condensate drain, a cracked or disconnected hose, a failed inducer motor, or a pressure switch that’s stuck open can all cause this. You can check the hose visually for obvious cracks, disconnections, or standing water inside it. Beyond that, proper diagnosis requires a manometer to verify the inducer is generating adequate static pressure, and that’s a tech job.

The flame sensor (second most common)

The flame sensor is a thin metal rod that sits in the burner flame. The board applies a small AC voltage to the rod; the flame acts as a rectifier, and the board reads the resulting DC signal in microamps. If the rod is coated in oxidation, it can’t conduct well enough for the board to trust that a flame is present, so the board closes the gas valve after a few seconds and the burner goes out.

This shows up as the burner lighting briefly, then shutting off before the system locks out. If that’s what you’re seeing, the flame sensor is almost certainly your issue. A tech will pull the sensor, clean the rod, reinstall it, and confirm the microamp reading with a multimeter. It’s about a 20-minute repair and usually inexpensive.

The gas valve

If the pressure switches are healthy and the flame sensor isn’t the problem, attention shifts to the gas valve. On a call for heat, the board sends 24 volts to the gas valve terminals to open it. A technician will verify whether that voltage is actually arriving at the valve. If it is and the valve still won’t open, the valve itself has failed.

Gas valve replacement is a job for a licensed contractor, specifically a C-20 HVAC or C-36 plumbing licensee in California. It involves disconnecting and reconnecting a gas line, which requires proper leak testing and procedure. Not a homeowner job.

One thing worth ruling out first: make sure the gas supply valve on the pipe going to the furnace is fully open, and check that other gas appliances in the house are working. A separate supply issue can look exactly like a failed valve at the furnace.

The igniter itself, one more time

A hot surface igniter that looks like it’s glowing might actually be operating below temperature. Igniters glow visibly well before they reach the temperature needed to reliably light gas. A cracked igniter or one drawing too little current can look fine to the eye and still fail to ignite the burner. If the pressure switch and flame sensor check out, a tech will check igniter resistance and surface temperature with calibrated instruments before condemning or replacing it.

What you can safely check yourself

A few things are reasonable to look at before calling anyone:

• The air filter. A completely blocked filter can cause the inducer to struggle, which can trip the pressure switch. Pull it out and look at it.
• The condensate drain line on a high-efficiency furnace. If it’s blocked, water backs up and can block the pressure port or trigger a float switch. The drain is usually a white PVC pipe running to a floor drain. Clear any visible blockage at the outlet.
• The gas supply valve and whether other gas appliances in the house are working.
• Whether the furnace is flashing an error code on a small LED on the board. Your furnace door usually has a legend for those codes. Count the flashes and have that number ready when you call.

Don’t probe electrical terminals, attempt to bypass safety switches, or touch the igniter with your hands (oil from skin degrades them).

When to call

If you’ve checked the filter, drain, and gas supply and the furnace still won’t fire, you need a technician. Pressure switch diagnosis requires a manometer. Flame sensor repair is fast with the right tools. Gas valve work legally requires a licensed contractor in California.

Most Bay Area homeowners with this symptom end up with a pressure switch or flame sensor issue. Both are inexpensive fixes when caught early. Waiting tends to make things worse, particularly if a condensate issue is spreading to other components.

Call us at (925) 999-4095, 7 days a week. We’ll get you on the schedule fast, often same or next day when we can. We’ll diagnose the problem and tell you exactly what’s wrong before recommending any parts or work.

Key takeaways

• A glowing igniter means the board is working and the sequence started; the failure is downstream.
• Pressure switch problems are the most common reason a furnace won't fire after the igniter heats up.
• A dirty flame sensor causes the burner to light briefly then shut off; a tech can confirm and fix it in about 20 minutes.
• Gas valve replacement requires a licensed C-20 HVAC or C-36 plumbing contractor in California.
• Check the air filter and condensate drain before calling a tech; both can trip safety switches.

Related questions

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA 608 certification, and completed factory training at the Daikin/Goodman plant in Houston in 2025. He writes from direct field experience, not marketing copy.

Further reading

• Mitsubishi Hyper-Heat (H2i): How It Keeps Putting Out Heat When It Gets Cold in the Bay Area , Most people think heat pumps quit when it gets cold. Mitsubishi's Hyper-Heat (H2i) line is built to keep working as temperatures drop, which matters more here than folks expect, especially on cold mornings and up in the East Bay hills.
• Mitsubishi Mini-Split Blinking a Fault Light? What It Means and What to Do , A blinking light on your Mitsubishi indoor head is the system telling you something is wrong. Here is what the blink pattern means, the checks you can safely do yourself, and when to bring in a factory-trained tech.
• Why Your Mitsubishi Mini-Split Is Leaking Water Inside (and How to Fix It) , A Mitsubishi mini-split dripping water inside almost always comes down to condensate that can't drain. Here's what causes it, the safe checks you can do yourself, and when to call a tech.