Furnace Blinking Light: What the Flashes Mean by Brand
By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the makers’ own manuals 
A furnace's blinking light is the control board counting out a code, but the counts are not the same on every board. Three flashes is a pressure switch on most boards, yet stuck open on some and stuck closed on a Honeywell S9200. A steady light is normal on Carrier and Goodman, and a failed control on a White-Rodgers 50A55. Read the brand first.
The small light on a furnace’s control board is how it tells you what went wrong. It counts out a number in flashes, or shows a code on a small display. The trouble is that the counting is not universal: the same three flashes, or the same steady light, means different things on different boards.
This page brings together what the makers’ own manuals say, as quoted on this site’s brand pages. It shows how to read the light safely, where the brands disagree, and which page has your furnace’s full list. Every brand-specific line here comes from the manual cited on that brand’s page.
Before you start
What to carry: a torch, a pen and your phone for a photo. Ten minutes.
Gas or electric? This page is about gas furnace control boards, natural gas or propane. Oil burners have their own controls: the Beckett GeniSys lights.
Find the brand and the board. The brand is on the furnace. On a replacement board, the board’s own maker and model are printed on it, and that is the list to use.
The chart on your own furnace wins. Look for a code label on or inside the door. Carrier, for one, points to the service label on the blower access door.
Flame with the gas off, or the rollout switch. Where a code points at either, turn the furnace and the gas off and call a technician: what a rollout switch is telling you.
If you smell gas, leave the house and call the gas company from outside.
How to count
Count it properly
A count and a pause. White-Rodgers says each flash on the 50M56U lasts about a quarter of a second and each pause about two seconds. Watch the code come round twice and make sure it matches.
Two digit counts. On Carrier, short flashes give the first digit and long flashes the second, so two short and three long is 23. Resideo’s S9200 pauses a second between the two numbers of a pair.
Colours. York uses green, amber and red, and the S9200 has separate amber, red and green lights, each with its own meaning.
Displays. Newer Trane, Lennox and Rheem furnaces show codes on a small display, such as E2.2, E223 or 45. Note every character: Trane’s E2.1 and E2.2 mean different things.
Stored codes. Some boards keep a history. York keeps the last five through a power cut and ICP keeps the last code, while Carrier clears its stored code after two or three days.
Same light, different meanings
| The light | What it means, by board |
|---|---|
| Steady on | Normal on Carrier, Goodman and the White-Rodgers 50M56U. A failed control on the White-Rodgers 50A55. A lockout that resets after an hour on ICP boards. A fault inside the control when the White-Rodgers 21V51U shows solid red. |
| 3 flashes | Pressure switch not closing on Goodman, Lennox ML180, York, ICP, the White-Rodgers 50A55, 50M56U and 21V51U, and the Honeywell SmartValve. Pressure switch stuck closed on the Honeywell S9200. |
| Rapid flashing | Reversed polarity on Goodman and the White-Rodgers 50M56U, reversed power or grounding on Carrier. Flame with no call for heat on the White-Rodgers 50A55. Low flame current when York’s amber light, or the White-Rodgers 21V51U’s amber light, flashes rapidly. |
| Red, heartbeat | A normal call for heat on the Honeywell S9200. |
What the common codes point at
Work the owner checks
Pressure switch codes. The vent outside for snow, ice or a nest, the drain trap and the small hoses: the places a pressure switch fault hides.
Limit codes. A dirty filter or closed registers overheating the furnace: why a furnace overheats and cuts out.
Ignition and flame codes. Is the gas on? Then the flame sensor, which a dirty rod upsets: how to clean a flame sensor safely.
Polarity and ground codes. Wiring, which is an electrician’s job.
A lockout. Several boards try again by themselves after an hour, among them the White-Rodgers 50M56U, Goodman’s and York’s.
Find your furnace’s full list
| Brand or board | How it shows codes | The full list |
|---|---|---|
| Carrier | Two digit flash codes | Carrier furnace codes |
| Trane | E codes on a display, or flashes on older ones | Trane furnace codes |
| Goodman | Flashes, or EE codes on a display | Goodman furnace codes |
| Lennox | Flashes, or E codes on a display | Lennox furnace codes |
| Rheem | Two digit codes on a display | Rheem furnace codes |
| York | Green, amber and red flashes | York furnace codes |
| Heil, Tempstar and other ICP brands | Short and long flashes | ICP furnace codes |
| White-Rodgers 50A55 board | Flashes | 50A55 codes |
| White-Rodgers 50M56U board | Flashes, with fault recall | 50M56U codes |
| Honeywell S9200 board | Amber, red and green lights | S9200 codes |
| White-Rodgers 21V51U board | Green, amber and red flashes | 21V51U codes |
| White-Rodgers 50M56X board | E codes on a display | 50M56X codes |
| White-Rodgers All-Spark 50D50U module | E codes on a display | All-Spark codes |
| Honeywell SmartValve SV9510, SV9520 | A heartbeat, flashes and 6 plus codes | SmartValve codes |
When the light is not the problem
Check what comes before the board
No light at all. Check the furnace has power first: the breaker, and the switch on or near the furnace: the furnace checks in order.
A normal light and still no heat. The thermostat may not be calling: when the thermostat is the problem.
After a power cut. Some furnaces need a moment, or a reset, when the power comes back: heat that does not come back after an outage.
Questions people ask
Why is my furnace light blinking?
The control board is counting out a code. Count the flashes, note any colour, and look the count up for your brand or board, because the meanings differ.
What does 3 blinks mean on a furnace?
On most boards, a pressure switch that will not close, which points at the vent, the drain or the hoses. On a Honeywell S9200 it means the switch is stuck closed instead.
Is a steady light normal?
On Carrier, Goodman and the White-Rodgers 50M56U, yes. On a White-Rodgers 50A55 it means the control has failed, and on ICP boards it is a lockout that resets after an hour.
What does rapid flashing mean?
On several boards it is reversed polarity or a grounding problem, an electrician’s job. On a White-Rodgers 50A55 it means flame with no call for heat, which is a stop and call.
Why did the code disappear when I opened the door?
The door switch cut the power, and many boards lose the current code with it. Read it through the window next time.
Is a red light always a fault?
No. On a Honeywell S9200, a red heartbeat is a normal call for heat. Colour only means something together with the brand’s own chart.
Sources: The furnace makers' own manuals, as cited on this site's brand pages: Carrier, Trane, Goodman, Lennox, Rheem, York, ICP (Heil and Tempstar), White-Rodgers 50A55, 50M56U, 21V51U, 50M56X and 50D50U, and Honeywell (Resideo) S9200 and SmartValve SV9510/SV9520
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