Carrier furnace Code 34 (3 short, 4 long LED blinks) is ignition proving failure: the furnace lit or attempted to light but the flame sensor did not confirm a flame. The most common fix is cleaning the flame sensor rod with fine steel wool or emery cloth. The flame sensor is a thin metal rod extending into the burner flame, and a thin oxide coating on the rod surface is enough to block the microamp signal.
Most Code 34 calls I get are just a dirty sensor. Takes five minutes to clean it and the furnace runs fine. But ignore it and the board goes into hard lockout, your house gets cold fast, and repeated failed ignition attempts stress the ignitor. What started as a $0 cleaning job turns into a $60 part swap real quick.
Phillips #2 screwdriver, Flathead screwdriver (small, for wire terminals)
What Does the 34 Code Mean?
Carrier units are notoriously picky about the microamp signal from the flame sensor. I cleaned three of these last week alone and honestly two of them just needed a quick polish with steel wool. Even a microscopic layer of oxidation on that rod is enough to block the signal and trigger lockout. Before you buy anything, spend five minutes cleaning the sensor. Clears this fault probably 70% of the time.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Oxide coating on flame sensor rod blocking microamp signal40%
Cracked or failed hot surface ignitor not reaching ignition temperature24%
Low gas pressure causing weak or no flame14%
Flame sensor wire disconnected or corroded at terminal12%
Gas valve opening too late or too briefly10%
Symptoms You May Notice
Ignitor glows orange, gas valve clicks, you see a flame for maybe two seconds and then the whole thing shuts off.
Control board LED cycling 3 short blinks then 4 long ones, over and over.
House isn't heating even though the thermostat is calling for heat and you can hear the furnace trying.
Furnace restarts itself every few minutes before going into hard lockout, usually after three failed attempts.
Lights fine on the first call of the day, then throws Code 34 randomly on the second or third cycle.
Can you reset a Carrier furnace to clear the 34 code?
Flip your thermostat to Off and wait a full minute before switching back to Heat. If the board's still in hard lockout, find the power switch on the furnace cabinet and toggle it off and on. Still stubborn? Pull the 3-amp fuse on the control board for about ten seconds to force a hard reboot. Just know the code comes right back if you haven't actually fixed the sensor or ignitor issue first.
Tools Required for Diagnosis
Phillips #2 screwdriverFlathead screwdriver (small, for wire terminals)Multimeter with DC microamps setting0000 steel wool or 220-grit emery clothNeedle-nose pliersNitrile glovesPaper towels
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range1.5–4 microamps DC
ConditionIf Open (OL) or infinite, replace component.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
Flame Sensor RodLH680016 · $10–$20
LH680016
$10 – $20
Hot Surface Ignitor (Silicon Nitride)HH18HA483 · $25–$50
HH18HA483
$25 – $50
Frequently Asked Questions
How do I clean a flame sensor on a Carrier furnace?
Cut the power at the breaker first, not just the thermostat. Give the ignitor five minutes to cool down. Find the flame sensor in the burner assembly, the thin rod with one wire on it, remove the single mounting screw and pull it out. Use 0000 steel wool to polish the rod until the dull oxidation is gone and the metal looks shiny. Wipe it clean with a paper towel. Don't touch the cleaned rod with bare fingers. Reinstall it, reconnect the wire, and test a heat cycle. Do this every fall before heating season and you'll rarely see Code 34.
What microamp reading is normal for a Carrier furnace flame sensor?
A healthy Carrier furnace should produce between 1.5 and 4.0 microamps DC. Most of these units start acting up and throwing Code 34 once the signal drops below 1.0 microamp. If your meter shows anything under 0.5, the control board shuts the gas valve off almost immediately because it can't prove flame is there. Cleaning the rod usually brings a marginal reading back above 1.5. If you cleaned it and you're still reading under 1.0, the sensor itself is defective or your ground connection at the mounting screw is corroded.
Why does my Carrier furnace light and then immediately shut off?
Classic Code 34 symptom. The furnace lights the gas but the control board has a narrow proving window, usually 7 to 10 seconds, to detect a microamp signal from the flame sensor. If the sensor is dirty or failing, the board thinks ignition failed and shuts the valve off as a safety measure. After three failed attempts in a row, the furnace goes into hard lockout for roughly three hours. Don't just wait it out. Go fix the sensor and reset the board.
How much does it cost to fix Carrier furnace Code 34?
If it's just a dirty flame sensor, your cost is zero dollars and about 20 minutes of your time. A replacement flame sensor for a Carrier runs $10 to $25 for the part. Hot surface ignitors are $30 to $60 depending on the model. If you're calling an HVAC tech, expect $85 to $150 for a service call and the tech will probably fix it in under an hour. I'd always try cleaning the sensor yourself first because that clears it most of the time.
Can a bad hot surface ignitor cause Code 34 on a Carrier furnace?
Yes, absolutely. If the ignitor isn't getting hot enough to light the gas, the valve opens but nothing ignites, so the flame sensor never sees a flame and the board throws Code 34. Watch it during startup. It should glow a bright orange-white within about 30 seconds. Dim red or nothing at all? That's your problem. Ignitors on these Carrier units typically last 4 to 6 years, and honestly the 59TP6 and 58PAV models burn through them faster than average in my experience.