Furnace Light Blinking?
Good news: that blinking LED isn't random. It's your furnace spelling out exactly what's wrong in code — and you can read it yourself in about two minutes.
Quick Answer
Count the blinks, then match them to the legend printed inside your furnace. The LED flashes in a repeating pattern (say, 3 blinks, pause, 3 blinks), and a sticker on the inside of the lower furnace door — or on the control board itself — translates that number into a specific fault. A slow steady blink usually means normal operation; a repeating count means a fault; on many models a rapid continuous flash means the furnace has locked itself out after failed ignition attempts.
Important: the exact codes differ by brand AND model — your furnace's own sticker is the authority, not a chart from the internet.
How to Read It, Step by Step
1. Find the LED
Look through the small clear window in the lower furnace door — the light is on the control board behind it. If there's no window, the lower door usually lifts off (the furnace shuts off when you remove it; that's a safety switch, not a new problem).
2. Count the pattern
Watch a full cycle: the light blinks a number of times, pauses, and repeats. Some boards use two colors (red and green) or long-short combinations — write down exactly what you see, like '3 red' or 'long-short-short.' That pattern is the fault code.
3. Find the legend
The translation is printed on a sticker inside the lower door panel, on the control board itself, or in the manual. It maps each count to a fault — pressure switch, limit switch, flame failure, and so on. This is why we don't print a universal chart here: a '3-flash' means different things on different boards, and guessing wrong wastes your evening.
4. Try one reset — and only one
Flip the furnace's power switch (looks like a light switch, on or beside the unit) off for 30 seconds, then back on. This clears a lockout and gives the furnace fresh ignition attempts. If it lights and stays running, a marginal sensor tripped it — worth a tune-up soon. If the same code returns, the fault is real.
5. Tell us the code when you call
Whatever you counted — '4 flashes on a Goodman,' 'blinking red on a Lennox' — say it on the phone. It tells our technician which parts to have on the truck before we ever pull into your driveway, and it often turns a two-visit repair into a one-visit fix.
What the common fault families mean
Across Carrier, Bryant, Trane, Goodman, Amana, Rheem, and Lennox boards, most codes land in a few families: pressure switch faults (often a blocked intake/exhaust pipe or a failing inducer motor), limit switch trips (usually a clogged filter overheating the furnace — check yours first), flame failures (dirty flame sensor or igniter — see the no-heat checklist), and lockouts after repeated failed starts. The code narrows it; the fix is usually a one-visit part swap.
The Brand-by-Brand Code Troubleshooter
Tap your brand to jump to its table. These are the widely published legends for each manufacturer's most common control boards — but codes DO vary by model and board revision, so the legend printed inside your own furnace door is always the final word.
Safety line, once and clearly: these tables are for reading the code, not for opening gas valves or wiring. Anything beyond a filter, a vent pipe check, and one power-cycle belongs to a technician.
Carrier / Bryant / Payne
Also Day & Night, Heil-family boards on some models
These brands share two-digit codes: SHORT flashes are the first digit, LONG flashes the second (3 short + 1 long = code 31). Applies to most models from the 1990s onward with the small viewing window in the lower door.
| Code | What it means | What to do |
|---|---|---|
| 11 | No fault stored — system considers itself normal. | If you still have no heat, run the no-heat checklist: thermostat, breaker, filter, gas valve. |
| 12 | Blower running after power interruption (normal for up to 90 seconds). | Wait a couple of minutes — this usually clears itself. |
| 13 | Limit circuit lockout — the furnace overheated repeatedly and locked out. | Replace the filter, open all vents, then power-cycle once. If it returns, call — repeated overheating damages the heat exchanger. |
| 14 | Ignition lockout — three failed attempts to light. | Power-cycle once for fresh attempts. If it locks out again it's usually a dirty flame sensor or failing igniter — same-day tech fix. |
| 21 | Gas heating lockout — control sees a gas valve fault. | Do not keep resetting this one. Call us. |
| 23 | Pressure switch did not open as expected. | Technician diagnosis — usually the switch or inducer motor. |
| 24 | Secondary voltage fuse is open — often a shorted thermostat wire. | Technician fix; mention if you recently installed a thermostat. |
| 31 | Pressure switch did not close — the #1 code we see. Blocked intake/exhaust pipe, failing inducer motor, or a bad switch. | Check the white PVC pipes outside for nests, leaves, or ice, then power-cycle once. Still 31? Call. |
| 33 | Limit switch tripped — furnace overheating, almost always airflow. | Replace the filter and open closed vents. If it repeats, the blower may be failing. |
| 34 | Ignition proving failure — burners lit but the flame sensor can't see it. | Classic dirty flame sensor. Quick technician cleaning; a staple of every tune-up. |
| 41 / 42 | Blower motor (41) or inducer motor (42) fault. | Motor-level problem — technician territory. |
Trane / American Standard
Red LED behind the lower-door sight glass
Applies to most Trane and American Standard furnaces with the integrated control's single red LED (XR/XE/XV series and similar). Slow steady flash = normal with no call for heat; fast flash = normal while heating. Counted flashes are faults:
| Code | What it means | What to do |
|---|---|---|
| 2 flashes | System lockout — the furnace gave up after repeated failed starts. | Power-cycle once for fresh attempts. If it locks out again, call — usually igniter or flame sensor. |
| 3 flashes | Draft pressure switch fault — venting or inducer problem. | Check the intake/exhaust pipes outside for blockage or ice, then one reset. Persistent = call. |
| 4 flashes | High limit open — the furnace is overheating. | Replace the filter, open all vents. Repeated trips deserve a same-day look. |
| 5 flashes | Flame sensed when there should be none. | Stop and call — this is a gas-side fault, not a DIY item. |
| 6 flashes | Reversed 115V polarity or poor grounding. | Electrical/installation issue — technician fix. |
| 7 flashes | Gas valve circuit error. | Do not keep resetting. Call us. |
| 8 flashes | Low flame sense signal — the sensor is barely seeing the flame. | Dirty flame sensor 90% of the time. Quick cleaning restores it; it'll die completely soon if ignored. |
| 9 flashes | Check igniter — failing or poorly grounded. | Igniter replacement is a common one-visit fix off the truck. |
| Solid red | Control board internal fault. | Power-cycle once; if it stays solid, the board likely needs replacement. |
Goodman / Amana
Also many Daikin-era boards; red LED on the control
Applies to most Goodman and Amana single-stage furnaces (GMS/GMSS/AMS families and similar). LED off = no power or failed board; steady on = normal. Counted flashes are faults:
| Code | What it means | What to do |
|---|---|---|
| 1 flash | Ignition failure lockout — several failed lighting attempts. | Power-cycle once for fresh tries. Repeats = dirty flame sensor or igniter; same-day tech fix. |
| 2 flashes | Pressure switch stuck CLOSED before the inducer starts. | Technician diagnosis — switch or inducer. |
| 3 flashes | Pressure switch stuck OPEN — venting, hose, or inducer problem. | Check outside intake/exhaust pipes for blockage or ice, then one reset. Persistent = call. |
| 4 flashes | High limit switch open — overheating, almost always airflow. | The dirty-filter special. Replace it, open vents, give it 30 minutes. |
| 5 flashes | Flame sensed with no call for heat. | Stop and call — gas-side fault. |
| 6 flashes | Rollout switch open OR reversed polarity (model-dependent). | A tripped rollout means flames left the burner area — do not reset repeatedly; call us. |
| 7 flashes | Low flame sense signal. | Dirty flame sensor — quick technician cleaning. |
| Continuous | Reversed line polarity (on many models). | Electrical/installation issue — technician fix. |
Rheem / Ruud
Integrated furnace controls with a single status LED
Applies to most Rheem and Ruud furnaces with the common integrated (UT-style) control board. Steady on = normal. Counted flashes are faults:
| Code | What it means | What to do |
|---|---|---|
| 2 flashes | Pressure switch open with the inducer running — venting or switch fault. | Check the outside pipes for blockage/ice, one reset, then call if it returns. |
| 3 flashes | Pressure switch closed when the inducer is off. | Technician diagnosis — stuck switch or shorted hose/wiring. |
| 4 flashes | High limit open — overheating from restricted airflow. | Replace the filter, open all vents. Repeats deserve a same-day look. |
| 5 flashes | Flame sensed with the gas valve off. | Stop and call — gas-side fault. |
| 6 flashes | Rollout or auxiliary limit open. | Flames left the burner area at some point — do not keep resetting; call us. |
| 7 flashes | Ignition lockout after three failed attempts. | One power-cycle for fresh tries; repeats mean flame sensor or igniter — one-visit fix. |
Lennox
SureLight boards — TWO LEDs that flash together or alternately
Most Lennox furnaces (SureLight control) use two LEDs read as a pair, which trips everyone up. The pattern of the PAIR is the code — and Lennox legends vary more by model than any other brand, so the door sticker matters extra here:
| Code | What it means | What to do |
|---|---|---|
| Both flashing slowly together | Normal — power on, no call for heat. | If you have no heat anyway, run the no-heat checklist — the furnace isn't hearing the thermostat. |
| Both flashing rapidly together | Normal — call for heat in progress. | If it runs then quits, see our short-cycling page. |
| Alternating slow | Low flame signal — sensor barely seeing the flame. | Dirty flame sensor — quick technician cleaning. |
| Alternating fast | Reversed line polarity or grounding fault. | Electrical/installation issue — technician fix. |
| One steady / one flashing | A specific fault — Lennox assigns these per model. | This is exactly the case where your door-panel legend is the answer. Read it, or tell us the pattern when you call. |
York / Coleman / Luxaire
Johnson Controls family — red and green LEDs on most boards
These brands share boards and use flash counts similar to Goodman's family: pressure-switch, limit, and ignition faults on counted red flashes, with green typically showing normal status. Legends vary by series — the door sticker rules:
| Code | What it means | What to do |
|---|---|---|
| Counted red flashes | A stored fault — commonly pressure switch (venting), high limit (airflow), or ignition lockout, per your model's legend. | Filter, vents, outside pipes, ONE reset — then read your sticker and tell us the count when you call. |
| Rapid red flashing | Lockout on many models. | One power-cycle clears it for fresh attempts; a repeat lockout is a same-day call. |
Brand not listed, or the code doesn't match?
Then your board uses its own legend — read the sticker inside the lower door, or just tell us the brand and the exact pattern when you call. "Four red flashes on a York" is enough for our dispatcher to load the right parts on the truck.
Ready for a pro to take it from here?
Call with the flash code and our tech loads the right parts before rolling — igniters, flame sensors, pressure switches, and control boards ride on every winter truck. Our heating repair crews run same-day calls across Central Arkansas — including heating repair in Bryant, heating repair in Benton and heating repair in Little Rock.
Read the Code? Tell Us What It Said.
$149 flat diagnostic, $75 credited to the repair, and same-day heating repair across Little Rock, Bryant, Benton, Conway, and Central Arkansas.
Call 501-307-5959