Your furnace clicks. You hear it try. It tries again. Then it gives up, the blower pushes room-temperature air around for a minute, and the house keeps getting colder. That sequence — try, quit, retry, lock out — usually points at two of the smallest, cheapest parts in the entire machine: the hot surface ignitor and the flame sensor.
Both are inexpensive. Both are common. And both get quoted wildly differently depending on who shows up. Here is the honest breakdown of what they do, how to tell them apart, and what a fair number looks like in Puget Sound.
What these two parts actually do
Every modern gas furnace runs a fixed safety sequence: draft inducer spins up, pressure switch confirms airflow, ignitor heats, gas valve opens, burners light, flame sensor confirms flame, blower starts. Any step that fails stops the whole chain. Two of those steps depend on parts the size of a finger.
The hot surface ignitor
On most furnaces built since the early 1990s, there is no pilot light. Instead a small silicon carbide or silicon nitride element glows orange-hot — hot enough to light gas on contact. It is brittle ceramic that gets heated and cooled thousands of times over its life. Eventually it cracks or loses conductivity and stops glowing. That is not a defect; that is the part doing its job until it is done.
Symptom: the furnace calls for heat, the inducer runs, and then nothing lights. No burner. No flame. Just a retry cycle and eventually a lockout code blinking on the control board.
The flame sensor
The flame sensor is a thin metal rod sitting in the burner flame. It does not light anything. It proves the flame exists by carrying a tiny electrical current through the flame itself — microamps, not volts. Over years of combustion, a fine oxide film builds on the rod. The current drops. The board decides there is no flame and shuts the gas valve within a couple of seconds, because a gas valve open with no flame is exactly how houses fill with unburned gas.
Symptom: the burners do light, you see or hear flame for two to five seconds, then it snaps off. Repeat three times, lockout.
That distinction is the single most useful thing a homeowner can report on the phone.
Quick diagnosis by symptom
| What you observe | Most likely cause |
|---|---|
| What you observeInducer runs, never any flame | Most likely causeIgnitor failed, or gas supply issue |
| What you observeLights, runs 2–5 seconds, shuts off, retries | Most likely causeDirty or failed flame sensor |
| What you observeLights and runs, but cold air from vents | Most likely causeBlower or thermostat set to ON, not a sensor issue |
| What you observeNothing happens at all, no sounds | Most likely causePower, breaker, thermostat, door switch |
| What you observeLights fine some days, not others | Most likely causeMarginal ignitor or sensor about to fail |
What ignitor and flame sensor repairs cost here
Typical furnace repairs in standard Puget Sound markets run $150 to $1,500, and $175 to $1,800 in premium markets like Seattle, Bellevue and Gig Harbor. Ignitor and flame sensor work sits at the bottom of that band. These are the cheap ones.
A flame sensor cleaning is often the least expensive thing a technician does all week — it is labor and a diagnostic, not really a part. A flame sensor replacement is barely more. An ignitor replacement is a low-cost part plus the visit.
The number moves for real reasons, not invented ones:
- Access. A furnace in an open garage is faster than one wedged in a crawl space in Puyallup or a closet in a Renton townhome.
- Part availability. Common ignitors are stocked. Some older or brand-specific parts have to be ordered, which means a return visit.
- What else is wrong. If the sensor is filthy because the burners are filthy, cleaning one rod and leaving the rest is not a repair, it is a delay.
- Market tier. Premium market pricing runs above standard, which is why the ranges above have two sets of numbers.
Check these before you call
None of this costs anything, and it occasionally saves you a service call entirely.
- Thermostat. Set to HEAT, fan on AUTO, setpoint at least three degrees above room temperature. Fresh batteries.
- Breaker. Furnaces sit on a dedicated circuit. Check the panel and the switch on the wall near the unit — the one that looks like a light switch and gets flipped by accident constantly.
- Furnace door. The access panel has a safety switch. If the door is not seated, the furnace will not fire. This is a real cause of "dead" furnaces.
- Filter. A clogged filter overheats the furnace and trips the limit switch, which mimics an ignition problem and destroys ignitors early through short cycling.
- Gas. Is the range lighting? If nothing gas-fired in the house works, the problem is upstream of the furnace.
If all five pass and the furnace still will not light or will not stay lit, it is time for a technician.
Why these parts die in January and not July
Puget Sound's mild marine climate means furnaces here idle for most of the year. Winter design temperatures across our service area range from 17°F in Olympia and Lacey to 29°F in Tacoma and Gig Harbor. When the first genuine cold snap hits, a furnace that had been cycling twice an hour suddenly runs hard, and a marginal ignitor that survived a mild December finally cracks.
That is why an annual tune-up matters more than the price suggests. Cleaning the flame sensor and checking ignitor resistance are five-minute tasks during a maintenance visit and a no-heat emergency in February.
When it is not worth fixing
Honest version: most of the time, it is. An ignitor on a ten-year-old furnace is a cheap repair on a machine with years left. Fix it.
The repair-or-replace line is simple. Under 15 years old and the repair costs less than half of a new system? Repair it. Beyond that, the math changes:
- Furnace replacement runs $4,000 to $8,000 installed in standard markets, $5,000 to $10,000 in affluent markets.
- A ducted heat pump runs $12,000 to $18,000 standard and replaces heating and air conditioning in one system. It can also qualify for PSE rebate money — up to $4,400 toward a qualifying heat pump when it replaces an existing electric or fossil-fuel system, with the exact amount depending on what you are replacing and what you install. Programs change, so we confirm your current eligibility and handle the paperwork. From April 2, 2026, PSE rebates require a PSE-certified installer, which Varsity is. The federal 25C tax credit ended December 31, 2025 and does not apply to any 2026 install.
If you want the full comparison before you decide, read heat pump vs. gas furnace in Western Washington or look at ducted heat pump options.
Want a starting number without a phone call? The instant estimate tool gives Good, Better and Best pricing in about 60 seconds.
How a repair visit actually goes
A technician diagnoses the problem and gives you a price before any work starts. If it is a sensor cleaning, that is what you pay for — not a system replacement pitch. If a part has to be ordered, we say so and schedule the return. Same-day fixes happen when the part is on the truck and the access allows it.
We repair all major furnace brands, not only the ones we install. Service hours are Monday through Friday, 8 to 6, with same-week scheduling in most of our 45-city service area.
Get it looked at
A furnace that lights and quits is not going to fix itself, and running it through repeated lockout cycles does not help anything. Call (253) 785-4622 or request an estimate and we will tell you which of the two parts it is — and what it costs — before we touch it.