Gas furnace repair in Naugatuck, Cheshire and Southington
Gas furnace won’t ignite? Igniter, flame sensor or gas valve
A gas furnace that will not ignite is almost always stopped at one of three parts. The hot surface igniter lights the gas, the flame sensor proves the flame is there, and the gas valve lets the gas through. The furnace itself tells a technician which one, because a modern control board runs those parts in a fixed order and stops at the step that failed.
This page explains that order, what each part does, how each one fails, and what our technician tests on a gas furnace repair call in Naugatuck, Cheshire or Southington. It does not tell you how to clean or replace any of them. Gas parts are not a homeowner job, and we will say why. If you smell gas, stop reading, leave the house, and call the gas company or 911 from outside.
Takeaways
- On a call for heat, one common ignition control runs a 32 second prepurge and then a 34 second igniter warm-up with the gas valve closed before any gas flows (Resideo S8910U instructions).
- A hot surface igniter works like a lightbulb filament and is fragile; oil from skin shortens its life, so nobody should touch it (Carrier).
- If the flame sensor detects no flame, it tells the control board to shut off the gas, which is why burners can light and then quit seconds later (Trane).
- A furnace gas valve holds two automatic valves that work independently, so one backs up the other if it fails (Resideo). It is never a homeowner repair. Call 203-756-5040.
How a gas furnace lights, step by step
A furnace built since 2010 lights with an electronic igniter rather than a standing pilot, according to Trane, and the control board runs the start-up in a fixed order. The timings below come from the installation instructions for the Resideo S8910U hot surface ignition module, a replacement control used on many furnaces. Your furnace’s own board may use different numbers, but the order is the same.
The thermostat calls for heat and the furnace clears itself
On a fan-assisted furnace the small combustion blower starts first. Once an air-proving switch confirms airflow, the module “first initiates a 32-second delay to allow system prepurge.” Nothing lights yet. That quiet half minute is normal, not a fault.
The igniter warms up with the gas still closed
After prepurge the module powers the hot surface igniter “for a 34-second warmup period, during which the gas control is closed.” This is the orange glow you can see through the burner window on many furnaces. No glow here is the first clue a technician looks for.
The gas valve opens for a short trial
“At the end of the warmup period, the gas control opens for the ignition trial time,” four or seven seconds on this module. The igniter stays powered for only the first second or four seconds of that trial, then switches off. Gas meets the glowing igniter and the burners light.
The flame sensor proves the flame and holds the valve open
Near the end of the trial, the flame rectification circuit checks for a burner flame. Once lit, the module’s flame sensing circuit “detects the flame current and holds open the gas control.” The flame is watched continuously for the whole call for heat.
The board locks out when no flame appears
“If flame is not sensed by the end of the timed trial for ignition, the gas control closes and the module locks out.” Some settings allow three tries with a purge between each; others lock out after one. Either way the furnace is doing its job, which is to never let gas flow without a flame to burn it.
Every no-ignite call lands on one of those five steps. Where the furnace stops is what tells the technician which of the three parts to test first.
What the igniter does and how it fails
A hot surface igniter is a small ceramic element that glows hot enough to light gas on contact. Carrier’s guide to furnace ignitors puts it simply: “These ignitors work like a lightbulb filament. When electricity flows through, the element heats up and glows to ignite the gas.” Without it, “the burners cannot light, and your furnace will not produce warmth.”
Like a filament, it wears out. Carrier says “Ignitors are fragile and operate at extreme temperatures, naturally degrading over time,” and that “Electrical fluctuations can burn out the heating element.” The signs Carrier lists are the ones we hear on the phone. The blower runs but the air is not warm. The furnace starts and stops without reaching the set temperature. There is no orange glow during the ignition sequence. Carrier adds that it “does not specify one universal replacement interval for all furnace ignitors,” so an igniter is replaced when it fails or tests weak, not on a calendar.
On a visit, the technician checks whether the board is sending power to the igniter at step 2, whether the element glows, and whether it is cracked or has a dull spot. The igniter is handled by its base and never by the element, because Carrier notes that “oils from skin can shorten its lifespan.” That is one reason this is not a part to pull out and look at yourself.
The American Heating position on igniters
We test the igniter on every no-ignite call before we condemn it, because a cold igniter and a board that never called for it look the same from the outside. Our furnace tune-up guide covers the igniter test that happens every fall. A weak one caught in September is a part on the truck; a failed one in January is a no-heat call.
Why a flame sensor makes the burners light and then quit
A flame sensor is a metal rod that sits in the burner flame and proves to the control board that the gas actually lit. Trane describes it as “a safety device that detects a flame,” and explains what happens without it: “If no flame is detected, it signals the control board to turn off the gas supply, preventing a gas buildup.”
That is why a furnace with a bad sensor lights and then dies a few seconds later. The burners are fine. The board simply cannot see the flame, so it closes the gas at the end of the trial. Bryant names the usual cause: “When coated in carbon or dust, it shuts the gas valve as a precaution.” On many control boards that failure has its own light signal.
The test for this part is the flame current the sensor produces while the burners are lit, measured in series with the sensor lead, plus the ground path the signal depends on. A low reading with a good flame usually means a coated rod, which is cleaned or replaced; a low reading with a lifting or lazy flame is a burner problem, not a sensor problem. Trane’s advice to homeowners is blunt, and we agree with it: “You should not attempt to clean or replace the flame sensor yourself.”
Does it light and quit, or never light at all?
Tell us which when you call. It points to a different part, and it tells the tech what to bring. If it sounds like something you can wait on, we will say so.
Why the gas valve is two valves in one
The gas valve on a furnace is not one valve. A typical combination gas control, such as the Resideo VR8215, holds “manual valve, two automatic operators, and pressure regulator” in a single body. Resideo describes the two automatic valves this way: they “function independently, allowing one to act as a failsafe for the other.” The first is solenoid operated and “opens on thermostat call for heat; closes when call for heat ends.” The second is diaphragm operated, “opens under control of the regulator; closes if gas or power supply is interrupted.” Together they “meet codes requiring dual safety shutoff.”
So when a gas valve fails, the furnace usually fails safe: the igniter glows, the trial runs, and no gas arrives. A valve can also fail because the board never sent it the 24 volt signal at step 3, which is a board or wiring fault, not a valve fault. That is the distinction a technician draws before anyone orders a valve.
For the valve, the technician measures the voltage at its terminals during the trial and the gas pressure on the supply and manifold sides, then confirms the valve opens when it is told to. Every manufacturer page we read says the same thing about this part. Bryant: “Never attempt to repair a gas valve yourself.” Carrier: “If you suspect a gas leak, do not attempt to troubleshoot the furnace yourself.” The only homeowner item near this part is the manual shutoff lever on the pipe. Even that is a question for the phone call, not a reason to go downstairs with a wrench.
If you smell gas, stop reading
Connecticut’s Public Utilities Regulatory Authority says not to use a telephone or flip switches near the smell, and if the odor is strong, “leave the premises immediately and tell others to leave.” Call the gas company from a neighbor’s house or outside. “They will come and make the area safe at no charge to you.” If a carbon monoxide alarm sounds or anyone feels ill, the CPSC says to “get outside to fresh air immediately, and then call 911.” Read the PURA gas safety page once now, before you need it.
Which part is it? Match what your furnace does
You do not need to open anything to use this table. Stand near the furnace during a call for heat and listen and watch for the sequence above. Then find the row that matches. The last column is what the technician checks first on a gas furnace repair call near you.
| What the furnace does | Where the sequence stopped | First part the tech tests |
|---|---|---|
| Small fan runs, then the furnace sits silent, with no glow and no flame. | Between step 1 and step 2 | Pressure switch and the board’s call to the igniter. Carrier: inducer runs but burners do not light “is likely an ignition or pressure switch issue.” |
| Fan runs, no orange glow, no flame, then the fan runs on cold. | Step 2 | Igniter: power at the element, then the element itself. |
| Orange glow, a faint click, no flame, then the glow goes out. | Step 3 | Gas valve: voltage at the valve during the trial, then gas pressure. Also the manual shutoff position. |
| Burners light, burn for a few seconds, go out, and the furnace tries again. | Step 4 | Flame sensor: flame current and ground. Bryant: a coated sensor “shuts the gas valve as a precaution.” |
| Repeated clicking, no flame. | Step 3 on a spark ignition system | Spark igniter and gas supply. Bryant: clicking “usually indicates the igniter or pilot light is trying to light the burners but failing.” |
| Three tries, then it stops and a light on the board blinks. | Step 5, lockout | The flash code first, then whichever part the code names. |
| Gas smell, or a CO alarm. | Does not matter | Leave the house. Gas company or 911 from outside. |
The table leaves out resistance readings for the igniter and microamp readings for the sensor, and that is on purpose. The national guides that rank for this question hand those numbers out, and they are the numbers a technician uses. They are also measured with the panel off and the furnace running, on a gas appliance. We do not print them for homeowners, and we would rather lose that reader than send one to the burner box with a meter.
What our technician tests on a gas furnace repair call in Naugatuck, Cheshire or Southington
The visit runs in the same order as the furnace. First the technician reads the light on the control board, because a board like the White-Rodgers control above “continuously monitor[s]” its own operation and the system and flashes a code for the fault it saw. Second, the thermostat is set to call for heat and the technician watches the sequence through the burner window, timing each step against the board’s own specification. Third, the part the sequence stopped at gets tested, in the way each section above describes: power and glow for the igniter, flame current and ground for the sensor, voltage and gas pressure for the valve.
Fourth, after the repair, the furnace is run through full cycles and the burner flame is checked. The Resideo module instructions call for exactly that at checkout: observe the lightoff, “make sure the main burner lights smoothly,” and confirm the burner “operates smoothly without floating, lifting, or flame rollout.” A flame sensor that reads low because the flame is lifting off the burner gets a burner cleaning, not a new sensor. That distinction is the difference between a repair that holds and one that comes back in February.
A Heil gas furnace with its two PVC vent pipes, beside an Electro-Air air cleaner cabinet, from our job library. The burner window is in the lower door.
Our shop is at 1114 Middlebury Rd in Watertown. Naugatuck is down Route 8, Cheshire is east on Route 68, and Southington is up Interstate 84, and we have serviced gas furnaces in all three since 1978. We answer 203-756-5040 around the clock, and if you are searching for gas furnace repair near me at two in the morning, that is the number. For the broader line between what a homeowner can touch and what a licensed tech should, our DIY versus professional repairs guide draws it room by room.
Here is one opinion from the truck. A furnace that failed to light once and then caught on the second try is not fixed. It is telling you a part is on its way out. Get it looked at while the weather is mild and the schedule is open.
Heat back on? The fall tune-up tests all three parts
Our tune-up guide covers what a real visit includes, what it costs in Connecticut and the best window to book it.
Frequently asked questions
Why does my gas furnace click but not light?
Clicking is the ignition system trying and failing. Bryant says a clicking sound usually means the igniter or pilot is trying to light the burners but cannot, most often because of a dirty flame sensor, a bad igniter or a gas supply problem. A technician watches the sequence to see which step the click belongs to.
My furnace lights, then shuts off after a few seconds. Is that the flame sensor?
Usually, yes. The flame sensor proves the flame to the control board, and Trane explains that if no flame is detected the board shuts off the gas to prevent a gas buildup. A rod coated in carbon or dust cannot see the flame, so the burners light and then quit. A tech measures the flame current to confirm it before replacing anything.
Can I clean the flame sensor or replace the igniter myself?
We say no, and so does Trane, which tells homeowners not to clean or replace the flame sensor themselves. Both parts sit in the burner compartment of a gas appliance, the igniter is fragile and damaged by skin oil, and the test that tells you whether a part is actually bad is done with the furnace running. Call us and describe what the furnace does instead.
How do I know if the gas valve is the problem?
You cannot tell from outside, and Bryant’s rule is never to attempt a gas valve repair yourself. The pattern that points there is an igniter that glows, a click, and no flame. A technician confirms it by checking for the control signal at the valve during the trial and measuring gas pressure, because a valve that was never told to open is a board fault, not a valve fault.
What do the flashing lights on the furnace control board mean?
The board flashes a code for the fault it detected. On the White-Rodgers 50A55 series, one flash is a system lockout and seven flashes is a low flame sense signal; other brands use their own tables. Count the flashes between pauses and tell the dispatcher the number. Do not reset the furnace over and over to clear it; the code is the clue.
Do you repair gas furnaces at night in Naugatuck, Cheshire and Southington?
Yes. We answer 203-756-5040 around the clock for heating emergencies, and all three towns are inside our service area from the shop in Watertown. We service gas and oil furnaces; if yours burns oil and keeps shutting down, read our oil furnace shutdown guide, since the burner and the reset button work differently.
Furnace will not light tonight?
Tell us what it does before it quits and how many times the board light flashes. Family owned in Watertown since 1978. Licensed in Connecticut, HTG.0386984 and SHM.0005083.
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About the author
American Heating and Air Conditioning Service
Family owned HVAC contractor, 1114 Middlebury Rd, Watertown CT 06795
We have installed and serviced gas and oil furnaces across the Greater Waterbury area since 1978, our technicians are NATE certified, and we answer the phone 24 hours a day for heating emergencies. Questions about anything in this article? Call us at 203-756-5040.
References and photo credits
- Resideo, S8910U Universal Hot Surface Ignition Module, Installation Instructions (form 69-0845-03)
- Resideo, VR8215 Standard Opening Direct Ignition Gas Valve
- Copeland, White-Rodgers Single Stage Integrated Furnace Controls 50A55-843 and 50A65-843
- Carrier, Furnace Ignitor: Understanding its Function and Importance
- Carrier, Furnace Not Turning On? Troubleshooting and Repair Guide
- Trane, Furnace Not Igniting? Find Out Why and What to Do
- Bryant, Furnace Not Turning On? 12 Causes and How to Fix It
- Connecticut Public Utilities Regulatory Authority, What To Do If You Smell Natural Gas
- U.S. Consumer Product Safety Commission, Carbon Monoxide Information Center
References checked October 14, 2026. General homeowner information, not a substitute for the instructions supplied with your equipment or an on-site diagnosis. Timings quoted are for the Resideo S8910U module; other control boards use their own values.
The hero is an editorial image generated on October 14, 2026 from one of the company’s own job photos; it is not a photograph of a specific customer’s home. All other photos are American Heating and Air Conditioning Service job photos from the company’s own library.
Heating services from American Heating
The pages behind this post, for when the furnace is the next job on the list.
- Furnace repair, service and installation across Greater Waterbury, gas and oil
- Heating services for the Greater Waterbury area, furnaces, boilers and more
- HVAC maintenance plans and service contracts, the fall tune-up that tests all three parts
- Heating system inspection and testing for a furnace that has been limping along
- Heating service in Naugatuck, repair, installation and maintenance
- Contact us for 24/7 emergency service, or call 203-756-5040
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