The short answer
If a Carrier furnace in Northern Virginia lights, runs for a few seconds and shuts the burners off, the flame sensor is the first suspect. It’s a thin metal rod in the flame that proves to the board the gas actually lit. A thin coating builds up on it over the years, the signal gets weak, and the board shuts the gas off to be safe. Carrier’s codes for it are 34, and then 14 once the board gives up.
What you’ll notice
- The burners light, then go out after a few seconds.
- The furnace tries again, and again.
- The light behind the lower door flashes 3 short and 4 long (34), or 1 short and 4 long (14).
- The house gets colder even though you can hear the furnace trying.
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What the codes mean
Code 34 is an ignition proving failure: the gas lit, but the board didn’t sense the flame. Carrier’s board tries three more times. If none of them prove, it moves to code 14, an ignition lockout, and waits 3 hours before it resets itself. Our code 34 post has the full code-by-code detail.
Why the sensor fails
The sensor works by sending a tiny current through the flame. Carrier’s manuals put the normal reading at 4.0 to 6.0 microamps, and the board needs at least 0.5 to call it a flame. A film of oxide and dust on the rod drops that number. So do a cracked ceramic insulator, a loose wire, a poor ground and a weak flame. Carrier also says the green and yellow ground wire has to connect to the furnace’s sheet metal, and the flame sensor itself must not be grounded.
Swipe to see all columns →
Swipe to see all columns →
| Cause | What it looks like | Who fixes it |
|---|---|---|
| A coated sensor rod | Dull or dark rod; the furnace lights and drops out | Us, or a careful homeowner with the power off |
| Cracked insulator or bad wire | Code 34 right after a cleaning | Us |
| Poor ground | Code 34 that comes and goes | Us |
| Weak or lifting flame | Burner flames that don’t hit the rod squarely | Us: gas pressure and burners |
| The board | A good flame signal and the board still drops out | Us |
How Carrier says to clean it
Carrier’s instruction is fine steel wool, rubbed lightly along the rod. We never use sandpaper or an emery board: they scratch the rod and leave grit on it, and the coating comes back faster. If you clean it yourself, turn the furnace off at its switch first, take out the one screw that holds the sensor, and handle it by the base. Don’t touch the igniter next to it: it’s brittle and a fingerprint can crack it. Put the sensor back the same way it came out.
What not to do
Never jump or bypass a safety switch to get heat back. Each one is there to stop the furnace from burning gas when something is wrong, and the wrong thing is often exhaust, which carries carbon monoxide. Don’t bend the rod to move it into the flame, and don’t keep resetting a furnace that drops out on 34. If the burners light and stay lit only when you stand there resetting it, something’s still wrong. If you smell gas, don’t flip any switches: leave the house and call your gas company or 911 from outside.
What a technician does
We read the flame signal in microamps before and after cleaning, so we know whether the cleaning worked or whether it’s the wire, the ground, the flame or the board. We watch a full ignition sequence, check the igniter and the gas pressure, and look at the burner flames. A sensor is a small part, and if it’s cracked or worn we replace it on the spot.
What it costs to have us look
A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before we touch anything. Our Carrier repair page lists what the common repairs cost. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler.
Related: flame sensor problems on any furnace, every Carrier furnace code, how to reset a Carrier furnace, the Bryant version and furnace won’t turn on. Other brands are on our error code and warning light guide.
Every code meaning here comes from Carrier’s own documents: 58SC0A installation instructions, 59SC6A installation instructions and 59MN7C manual.
Call or book
Call (703) 997-9757 and tell us the code you counted, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
How do I know if my Carrier flame sensor is bad?
The burners light and go out a few seconds later, and the light flashes 34 or 14. A technician can confirm it by reading the flame signal in microamps.
How do you clean a Carrier furnace flame sensor?
With the power off, remove the sensor and rub the rod lightly with fine steel wool, which is what Carrier’s manuals call for. Skip sandpaper and emery cloth: they scratch the rod.
What is a good flame sensor reading?
Carrier’s manuals list 4.0 to 6.0 microamps as normal, with 0.5 as the minimum the board accepts.
Why does my flame sensor keep getting dirty?
It sits in the flame, so a film builds up over time. If it fouls quickly, the flame, the gas or the combustion air needs a look.
How much does it cost to fix a flame sensor?
The visit starts with a flat $99 diagnostic, and you get the price of the repair before any work starts. Our heating check inspects the flame sensor, so a weak one gets caught before it quits.
