Flame Sensor Cleaning & Replacement

$185 flat-rate · Same-day Boone County · 2-yr labor warranty

What the flame sensor does

The flame sensor is a single metal rod (usually a Kanthal alloy) that protrudes into the burner flame path. When the gas valve opens and burners light, the flame ionizes air around the rod, creating a small electrical current (microamps, in the 1-10 µA range). The furnace control board detects this current and confirms "flame is present, keep gas valve open."

If the rod is fouled with combustion residue (typically a white-gray oxide layer that builds up over a season or two), the ionization current can't pass through the deposits. The control board sees "no flame present" even though the burners are clearly burning. Safety logic says "gas valve must be open with no flame — that's a leak — shut it off NOW." The board closes the gas valve, ends the cycle, waits, tries again. After 3 failed cycles, lockout.

Why it's the #1 furnace failure

• Cumulative residue. Combustion always produces some residue. Even clean-burning natural gas leaves microscopic deposits. They build up gradually over heating seasons.
• The sensor's location. It sits directly in the flame path. That's its job — but it's also the most thermally and chemically aggressive environment in the furnace.
• Most homeowners never know about it. Flame sensors weren't part of furnaces pre-1990. Many homeowners think their old furnace "just kept running forever" because there was no flame sensor to foul.
• It's covered by zero maintenance plans. Most "tune-ups" from competing companies don't include flame sensor cleaning. Ours does.

What's in the $185 flat rate

• $129 diagnostic confirming flame sensor is actually the failure (vs igniter, vs gas valve, vs pressure switch, vs cracked heat exchanger — all of which can present similarly)
• Sensor removal with proper handling (the ceramic insulator at the base is brittle and gets replaced if cracked)
• Cleaning with fine emery cloth until the metal is shiny clean
• Replacement with an OEM-spec part if the sensor is pitted, cracked, or worn beyond cleaning (about 30% of cases — common in older furnaces or systems where cleaning has been done multiple times)
• Test cycle with calibrated microamp meter verifying signal is in healthy range (2-7 µA typical)
• 2-year labor warranty

The DIY option (honestly)

If you're mechanically confident, this is one of the most accessible DIY fixes in HVAC. Procedure:

1. Kill power at the furnace switch (the light-switch-looking thing near the unit). Wait 30 seconds.
2. Remove the lower access panel (usually a hex screw or two thumbscrews).
3. Locate the flame sensor — single metal rod, about 4 inches long, mounted with one screw to the burner assembly, with a wire connecting to the control board. It'll be inside the burner shield, sticking into the flame path.
4. Note the wire connection. Disconnect carefully.
5. Remove the single mounting screw. Pull the sensor straight out.
6. Clean the metal portion with fine emery cloth, Scotch-Brite pad, or fine steel wool. Don't use sandpaper (too aggressive) or steel wool that leaves residue. Don't bend the rod.
7. Reinstall in reverse order. Tighten the mounting screw firmly but not over-torqued (the ceramic insulator cracks if over-tightened).
8. Restore power, set thermostat to call for heat, watch for normal cycle.

Takes 10 minutes start to finish. New flame sensors are $15-$30 at any HVAC supply house if cleaning doesn't restore function.

When to skip DIY and just call us:

• You don't know which part is the flame sensor and want someone else to find it
• You've tried cleaning and it didn't work — that means the issue is elsewhere
• The furnace is 20+ years old and combustion analysis is wise anyway
• You'd just rather not deal with it for $185

Why we still charge $185 when DIY works

Honest answer: because diagnostic + drive time + truck overhead. The actual cleaning is 10 minutes. But the $129 diagnostic includes verifying it's actually the flame sensor (not a misdiagnosis), checking gas pressures and combustion quality (worth doing anyway), and the truck stop costs us about $40 in fuel/wear regardless of job duration. $185 covers all of it.

Squadron Plan members ($189/yr) get flame sensor cleaning included in their annual fall tune-up. Effectively zero per-incident cost.

Frequently Asked Questions