How much does flame sensor cleaning cost?
$149 to clean it (most calls). $249 if it needs full replacement. Cheapest legitimate fix in HVAC — and 80% of 'furnace won't stay lit' problems are this.
San Antonio · Flame Sensor Cleaning & Replacement Repair · Family-Owned Since 1996
Furnace Lights, Runs 5 Seconds, Then Quits. That's The Flame Sensor — Cheapest Fix In HVAC.
The flame sensor is a thin metal rod that confirms your burner is lit. When it gets coated with carbon, it can't see the flame and the safety kicks in. Usually a cleaning, sometimes replacement.
Typical price
$149–$249
Time to fix
30–60 minutes
Urgency
High in winter — furnace can't sustain a fire
In plain English — what's going on with your flame sensor cleaning & replacement
When your gas furnace lights, the gas valve opens and stays open only as long as the flame sensor confirms there's actually a flame burning. The sensor is a thin metal rod stuck right in the burner flame — it passes a tiny electrical current through the ionized flame back to the control board, proving the burner is lit.
Over time, carbon buildup coats the rod and blocks that current. The board sees no flame signal, assumes the burner didn't light, and shuts the gas off as a safety. That's why your furnace lights, runs 5–10 seconds, then quits — and tries again, and quits — until it locks out.
Most calls we just clean it ($149) and you're back in business. Sensors do eventually wear out though — if cleaning doesn't restore the signal, we replace the sensor ($249). We always clean it as part of annual maintenance to prevent the no-heat call in February.
What it costs in San Antonio
$149–$249 — $149 to clean (most common). $249 if sensor needs replacement. Diagnostic included. Our $59 diagnostic is waived when you approve the repair, so you only pay for the fix itself.
Symptoms you'll notice at home
If two or more of these sound familiar, it's probably your flame sensor cleaning & replacement.
Furnace lights, runs for 5–10 seconds, then shuts off
Tries to light 3 times then locks out (LED error code)
Worked fine last winter, won't run this year
Recent maintenance was skipped
Black soot visible on the sensor rod
LED on control board flashes 'flame sense' code
Why flame sensor cleaning & replacements fail in San Antonio
- Carbon buildup on the sensor rod — normal over 1–2 winters.
- Sensor cracked or corroded beyond cleaning.
- Loose sensor wire or connection.
- Sensor mounted wrong distance from the flame (recent install issue).
- Sulfur or contaminants in the gas supply coating the sensor faster.
How we diagnose and replace it
Here's exactly what a Carnes and Sons tech does at your house — no mystery, no upselling.
- 1
Watch the ignition sequence
Confirm the burner lights but goes out after 5–10 seconds — that's the flame sensor fingerprint.
- 2
Test microamp signal with a meter
Healthy sensors pass 2–5 microamps when the flame is burning. Below 1 microamp means cleaning or replacement.
- 3
Clean the sensor rod
Remove sensor, gently clean with fine emery cloth or steel wool (never sandpaper — it leaves residue). Reinstall and re-test.
- 4
Verify reliable operation
Run furnace through 3 complete cycles. Signal should stay above 2 microamps and burner should stay lit. We test before we leave.
What Your Neighbors Are Saying
Get Your Flame Sensor Cleaning & Replacement Diagnosed Today
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Flame Sensor Cleaning & Replacement questions San Antonio homeowners actually ask
Why does my furnace light then shut off?
Almost always a dirty flame sensor. The sensor can't confirm the flame is there, so the control board shuts the gas off as a safety. Cleaning the carbon off the sensor rod fixes it in one visit.
Can I clean it myself?
If you're handy and comfortable shutting off gas and removing components, yes. Use fine emery cloth (not sandpaper). Don't touch the cleaned rod with bare fingers (oil contaminates it). If you're not sure, a $149 service call is cheap insurance.
How often should it be cleaned?
Every year as part of fall heating maintenance. Once a year and you'll never have a winter no-heat call from a dirty sensor.
Why does it get dirty so fast?
Combustion creates carbon as a byproduct. The sensor sits right in the flame so it accumulates that carbon over each winter's run hours. Some gas supplies (high sulfur) coat sensors faster.
Will replacement last longer than cleaning?
Same cleaning interval — even a brand-new sensor needs cleaning every year. Replacement is only needed when the rod itself is cracked or corroded beyond what cleaning can fix.
What if cleaning didn't fix it?
If the sensor reads above 2 microamps after cleaning but the furnace still shuts off, it's a different problem — control board, gas valve, or wiring issue. We diagnose those at the same visit.
Is this dangerous?
No — the flame sensor failing safe (shutting off gas) is exactly what's supposed to happen. The danger would be if the safety didn't work. Dirty sensor causing nuisance shutdowns is annoying but completely safe.
Serving flame sensor cleaning & replacement repairs across the greater San Antonio area
We're based in Stone Oak and roll trucks across San Antonio and the surrounding Hill Country every day.
Carnes and Sons Air Conditioning
License TACLB29435E · Family-Owned Since 1996
401 E Sonterra Blvd Suite 375
San Antonio, TX 78258(210) 600-5091
San Antonio, TX 78258(210) 600-5091
Mon–Sat: 7AM – 8PM · Emergency same-day service
Cities & neighborhoods we serve
San Antonio · Stone Oak · Alamo Heights · Hollywood Park · Hill Country Village · Shavano Park · Castle Hills · Leon Valley · Helotes · Boerne · Fair Oaks Ranch · Bulverde · Timberwood Park · Garden Ridge · Schertz · Cibolo · Selma · Universal City · Live Oak · Converse · Windcrest · New Braunfels
Talk to a real person about your flame sensor cleaning & replacement
No call center. The owner or one of his sons picks up.
(210) 600-5091