What a furnace that lights and drops out is telling you in Waco
A furnace that clicks and never catches, or that lights for a few seconds before the flame disappears, is describing two different points in the same sequence. Furnace clicking but not starting usually means the igniter never got hot enough or never sparked long enough to prove itself to the board. A furnace that fires and then drops is further along, past ignition and into the point where the flame has to prove itself to a sensor before the gas valve is allowed to stay open. Pilot light wont stay lit is the same complaint in older language, on equipment that traded a standing pilot for a hot surface or intermittent spark igniter.
The flame sensor sits in the burner flame and passes a small current back to the control board, and that current is the only proof the board has that fuel is burning rather than merely flowing. Furnace ignitor problems get blamed first because the igniter is the visible glow, but a coated sensor reads the same low signal a dead igniter would, and the board reacts to both the same way. Furnace not firing up on one attempt and firing on the next is a sign the sensor signal is marginal rather than absent, coming and going as a thin layer builds and partly burns off.
Waco heating season runs mild for most of its length, so a furnace can sit lightly used for weeks before a hard freeze asks it to hold flame every cycle overnight. That jump from occasional use to constant cycling is what exposes a sensor that was already coated or an igniter that was already thinning, and it tends to happen on the exact night the house needs heat most rather than during a mild stretch when a marginal part could limp along unnoticed.
Left alone, a coated flame sensor does not clear itself. The burner lights and drops out again and again as the board keeps trying to prove a flame it cannot confirm, and each failed attempt spends one of the tries the board allows before it stops. Once the trial limit is reached, the board locks out on repeated trials and stays there until it is reset, which means the house goes cold overnight with nobody aware until the temperature has already dropped for hours. That lockout is the point where a call the same evening keeps the house from being cold since midnight.
More detail on thermostat not working is on its own page.
Find the sign you have, then read across to see whether it can wait.
| Symptom or first sign | What is already happening | What it becomes if ignored | When to call |
|---|---|---|---|
| A coated flame sensor | The burner lights and drops out again and again | The board locks out on repeated trials and the house goes cold overnight | At the next visit, while the fix is small |
| A slow refrigerant leak | The system runs longer for less cooling and oil stops circulating properly | The compressor loses lubrication and seizes | This week, while it is still one part |
| A loaded filter | Airflow drops and the coil runs colder than it was designed | The coil ices over and the compressor takes liquid refrigerant back and fails | This week, while it is still one part |
How we read the ignition sequence before we quote a part
Diagnosing a furnace that will not hold flame follows a fixed order, and skipping ahead to a part is how a good igniter gets replaced for nothing. First the ignition sequence itself gets watched start to finish, the igniter glowing or sparking, the gas valve opening, and the flame either catching or failing to. That single watch tells whether the failure sits at ignition, meaning nothing lights, or at flame proving, meaning it lights and then drops. Furnace not firing up at all points toward the igniter or the gas valve. Lighting and then dropping points toward the flame sensor and the signal it sends back to the board, which is a different repair even though the symptom at the register sounds the same to whoever is standing in the hallway.
The flame sensor signal is read in microamps while the burner is running, and that number is checked against the point where the control board treats the flame as absent and closes the gas valve. A steady signal sitting well above that dropout point clears the sensor and points back at the igniter or the gas valve for further testing. A weak or drifting signal means the sensor is coated even if the flame is visibly burning, and cleaning it with a light abrasive pad and rechecking the same reading is the correct next step, not a replacement part. Reading the number before opening a parts box is what keeps a working igniter off the invoice, and it is the step a rushed lockout call skips most often.
A coated flame sensor, a worn igniter and a failing control board are all parts, and each gets replaced on its own once the reading points at it. A cracked heat exchanger is a different kind of decision, because it lets combustion gas mix with the air moving through the house, and finding one ends the visit with the furnace shut off rather than repaired that same day. The same logic applies when a furnace has needed the igniter or sensor serviced more than once in close succession, since repeated part failures on aging control components can be a sign the board itself is marginal rather than the sensor being unlucky twice in one winter.
If this sounds close but not quite your situation, ac not turning on may be the better starting point.
Your local utility, Utility Bill City of Waco, publishes service and water-quality information every Waco homeowner should know.
Ask which of these readings was taken before any part is replaced.
| Instrument | Good reading | Bad reading | What it changes |
|---|---|---|---|
| Flame sensor signal check in microamps | A steady signal well above the control board dropout point | A weak or drifting signal | The sensor is coated and the burner will drop out |
| Two probes for delta T between return and supply air | A 16 to 22 degree drop across the coil in cooling at normal indoor humidity | A small drop means airflow or charge is wrong | An unusually large drop means the coil is being starved of air |
| Clamp ammeter on the compressor and the blower | Running amps at or below the rated load amps stamped on the data plate | Amps steady above the rating, or a brief spike toward locked rotor on start | A failing capacitor, contactor or compressor |
| Micron gauge during evacuation | Below 500 microns and holding there after the pump is valved off | A reading that climbs back up | Moisture is still boiling off inside or the system is not sealed |
| Manifold gauges with superheat and subcooling readings | Superheat and subcooling both landing in the range the equipment maker specifies for that metering device | High superheat with low subcooling | An undercharge or a restriction rather than a weak compressor |
How an ignition call runs
The call and the history
On the phone we determine what symptoms showed up: a click with no flame, a flame that lights and drops, or nothing at all, and whether the furnace tried and locked out or just went quiet. We ask if power was interrupted recently and roughly how old the furnace is. On site the order is fixed: confirm the thermostat is calling for heat, confirm gas and power are reaching the unit, then watch a full ignition cycle before touching anything, so no part gets pulled before the actual failure point is seen.
Testing and the decision point
Once the cycle is watched, the flame sensor signal gets checked in microamps against the point where the board drops out, and that reading decides whether we are cleaning a sensor or testing the igniter and gas valve next. Whatever it points to gets written up as a scope before anything is replaced, and you approve it before parts go on. If the burner compartment shows scorching or a heat exchanger seam that needs a closer look, we stop and show you rather than close the panel and quote around it.
Verification before we leave
Before the visit ends, the furnace is run through a full ignition cycle again and the flame sensor signal is rechecked to confirm it holds well above the dropout point rather than just for one attempt. We walk you through what the reading was before and after, and you keep that number along with whatever part was replaced. If the fault was a coated sensor rather than a failed igniter, you see why the cheaper fix was the correct one.
Why a Waco furnace call is not the same job every month of the year
Waco heating season runs mild for most of its length, broken by short hard freezes rather than a long steady cold stretch. That pattern is what wears out ignition components unevenly. A furnace that sat mostly idle through a mild December can be asked to hold flame every night once a freeze sets in, and a sensor or igniter that was already marginal fails on exactly the night it is needed most. A crew unfamiliar with that rhythm treats every ignition failure as random wear, when the real driver is long idle stretches followed by sudden hard use.
Furnace location matters just as much on these calls. In the ranch and post war stock common around here, the furnace usually sits in a hall closet or a garage with the ductwork running through the attic above it, which means the access to the burner compartment is tight and the attic space above it runs hot for most of the year. A technician working that closet has to get a meter and a flame sensor tool into a space built for a much smaller service footprint than modern equipment gets, and rushing that access is how a sensor gets bent or a wire gets pinched during a swap.
The mistake an unfamiliar crew makes on this exact job is treating a locked out board as proof the igniter failed, ordering the part, and finding on return that the burner still drops flame because the sensor was never checked. That second visit is the one the reading in section two is built to prevent. On the initial call, a technician checks the sensor with a meter before ordering any parts. The sensor sits in the flame path where combustion heat reaches it, and a reading below the manufacturer threshold tells you the sensor has drifted or begun to fail. A bent sensor or one fouled with carbon will read low. Checking it first means you leave the site with either a working system or a single return trip to swap the sensor, not a parts order followed by a call back because a new igniter did not solve anything.
See furnace blowing cold air for how the related work is scoped and priced.
Permit and inspection questions? Commercial Construction Permits is the office we coordinate with on every permitted Waco job.
What working with us looks like on a call like this
Getting someone to the closet
When you call about a furnace that wont ignite, we ask what you are hearing at the burner and whether the board has locked out, and that tells us if this is a same day visit or one we can schedule for tomorrow. You get a window set before the technician leaves the prior job, not a guess made that morning. If a lockout call ahead of yours runs long because of a tight closet or attic access, we call to move your window rather than leave you waiting past it.
Working inside a tight closet
Furnace closets and attic platforms are not built for a technician kneeling with a meter and a flame sensor tool, so we clear the space around the unit before opening the burner compartment and lay down protection on the closet floor and any nearby carpet. Panels and screws come off in the order they go back on, kept together rather than scattered, and the closet gets swept and closed the way it was found before we walk you through what the reading showed.
What moves the cost here
On an ignition call the biggest swing in cost is the time to reach the unit. A furnace in an open garage takes less time than one behind a low attic platform or a packed hall closet, and that rigging time is what the scope reflects. Whether the fix is a cleaned sensor, a replaced igniter, or a control board also gets weighed before anything is added to the invoice. You see the scope, built from the flame sensor reading, and approve it before any part is ordered.
Who is trusted with the burner
A technician runs a set number of ignition and lockout calls alongside a lead before working a burner compartment alone, and the lead signs off on reading a flame sensor signal correctly rather than guessing from a visible glow. On a furnace with a scorched compartment or a heat exchanger that needs a closer look, a second technician confirms the reading before the furnace is shut off or a scope is written.
You Have Questions,
We Have Answers
Check the thermostat, the gas shutoff valve, and the furnace switch before anything else. Confirm the thermostat is set to heat and at least five degrees above room temperature. Look at the blower door switch, since a panel that is not fully seated cuts power to the whole unit. Replace a clogged filter, because restricted airflow can trip the high-limit switch. Leave the burner compartment closed. Gas valve and board work involves combustion gas and should stay with a technician.
Igniter replacement cost depends on the furnace make, igniter type, and burner compartment access. A silicon carbide igniter and a silicon nitride igniter are different parts with different prices. Cost climbs when the furnace sits behind a low platform or the board needs testing too. A flame sensor cleaning costs less than either. The scope gets approved before any part is ordered.
Turn the furnace power switch off for about thirty seconds, then switch it back on. That clears a lockout on most boards and restarts the ignition trial count. The igniter itself has no reset button. Older boards may also need the thermostat dropped and raised again. If the furnace locks out a second time, the reset is only hiding a coated sensor, a weak igniter, or a gas supply fault, so stop resetting.
A hot surface igniter often fails between three and seven years in service, though results vary. Silicon carbide types are brittle and crack from handling or vibration. Each cold start heats the element to over 2,000 degrees Fahrenheit, so frequent cycling thins the material. Touching the element with bare fingers leaves skin oil that creates hot spots and shortens its life. A sign of aging is a slower glow before the gas valve opens.
Wait through one full lockout cycle, roughly two to three failed attempts, then call. Most boards try three times before shutting the gas off. Clicking with no glow points to a dead igniter or a spark module. Clicking with a brief flame points to the sensor. Do not keep resetting the power, because each repeat sends more raw gas toward the burner. If the house is already cold or you smell gas, call immediately.
A furnace that lights and drops out usually has a flame sensor that cannot confirm the flame. Carbon or oxide film insulates the metal rod, so the current returning to the board falls below its threshold. The board then closes the gas valve within a few seconds as a safety response. Weak gas pressure, a dirty burner, or a poor ground can cause the same shutoff. A microamp reading at the burner separates these causes quickly.
Finding the cause usually takes under an hour when the burner compartment is reachable. Watching one full ignition cycle takes a few minutes, and the sensor reading takes a few more. Time grows when the furnace sits behind attic ductwork or a tight platform, or when a gas pressure check and board testing are both needed. A cracked heat exchanger inspection adds time because the burner assembly may have to be removed for a proper view.
Have the furnace model and serial number ready, found on the rating plate inside the blower door. Know whether the unit clicks, glows, lights briefly, or stays silent. Note how many times it tried before stopping and whether any error code is flashing on the board's small window. Check the filter condition and any breaker trips. Mention any smell of gas, since that changes the order of the response entirely.
Granite HVAC Waco reads the flame sensor signal in microamps before ordering any part. That keeps a working igniter off the invoice. Every scope is written from that reading and approved by you before parts go on. Each visit ends with a repeat ignition cycle and a recheck of the sensor signal, and you keep the before and after numbers. Call (254) 221-8461 to describe what the burner is doing.
Why Waco's mild winters make ignition parts fail unevenly
Waco winters are mostly mild, broken by short hard freezes rather than sustained cold. A furnace idles through the mild stretch and then operates continuously once a freeze sets in, and that sudden shift in duty cycle exposes a flame sensor that was already coated or an igniter that was already thinning. In furnaces sitting in a hall closet or garage with attic ductwork above, the same freeze that demands more heat also means the attic space the ducts run through has spent months absorbing summer heat, which creates thermal stress on the ductwork itself. The temperature swing between attic and conditioned space can also cause condensation in supply lines during the warmup cycle, adding moisture that corrodes ignition components faster than in climates with steadier winter temperatures.
Waco's hard freezes bring another local pattern worth knowing before an ignition call gets treated as routine. When a winter storm knocks out power and it comes back on all at once across a block, the surge that follows can trip a control board or scramble the memory it uses to count ignition trials, so a furnace that locked out that same night is not automatic proof of a failing igniter or sensor. Checking whether a recent outage lines up with the lockout changes the order of the visit, because a board reset gets tried and the flame sensor signal gets read again before any part is condemned. Furnaces sitting in a hall closet or a garage feel that surge the same way equipment anywhere else does, so the timing of the call against the last outage matters more here than the furnace's age or how long it sat idle beforehand.
HVAC Services in The Waco Area
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Address:
Granite HVAC Waco, Waco, TX, 76701
Checking that gas is turned on, the thermostat is set above room temperature, and the filter is not blocked is genuinely a homeowner job before calling. Where it stops is the burner compartment and anything involving the gas valve, the control board or the flame sensor, because that is powered equipment handling combustion gas. If what we find points at a cracked heat exchanger, we say so on the spot and tell you the furnace needs to stay off rather than restarted.
Before you call, know whether the furnace clicks with no flame, lights and drops, or stays silent, and when that pattern started. Note whether the power went out recently or the thermostat was changed just before it happened. If you can, find the furnace's age or the sticker inside the closet door. Those answers usually tell us whether this needs someone today.
Last reviewed September 2026 by Dale Whitfield, Service manager.