The short answer
A blinking light on a furnace in Northern Virginia is the control board talking. A steady glow or a slow, even blink means it has power and sees no fault. A pattern of quick flashes, a pause, then the same flashes again is a fault code: count it, match it to the chart on the inside of the furnace door, and it tells you which step of the startup failed. Most codes point at airflow, venting or ignition. A few mean turn the furnace off and call.
Where the light is and how to count it
Look through the small window in the lower door of the furnace. That’s the status light on the control board. Don’t pull the door off to get a better look. The door presses a safety switch, and on most boards cutting the power clears the stored code, so you lose the one clue you had.
On a Carrier or Bryant board the code is two digits: three flashes, a pause, then one flash is code 31, which means the pressure switch didn’t close. Other brands use a single count of flashes, or a color. Note the color, count two full rounds, and take a 30 second video on your phone. That video saves a lot of back and forth when you call. So does the furnace’s age, which is coded into the serial number on the same label: here’s how to read a Goodman or Amana serial number.
If you have a communicating thermostat, like a Carrier Infinity, Trane ComfortLink or Lennox iComfort, the same code shows on the thermostat screen in words. Start there. Our guide to thermostat messages covers the rest of what a screen can say.
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Which lights are normal
A steady light, or a slow blink about once a second or slower, is the board saying it has power and no faults. A rapid flicker means different things by brand. On many Carrier and Bryant boards it means the line voltage polarity is reversed, which usually shows up right after electrical work. On many Trane and American Standard boards, a slow green blink is normal, a fast green blink means the thermostat is calling for heat, and amber or red means a fault.
Your brand’s chart is the final word. We’ve written up the common ones: Carrier codes, Goodman codes, a Lennox flashing red light, Trane error codes, Rheem codes and Ruud codes.
The system we put in first
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What the codes usually mean
Every brand numbers them differently, but almost every code lands in one of these families.
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Swipe to see all columns →
| Code family | What it usually means | What you can check |
|---|---|---|
| No light at all | No power to the board | The furnace switch (it looks like a light switch on or near the unit), the breaker, and that the door is seated all the way |
| Pressure switch or venting | The inducer fan couldn’t prove the vent is clear | Snow or leaves over the white PVC pipes outside, a cracked rubber hose on the switch, water sitting in that hose. A weak inducer motor is ours to test |
| Limit switch open | It got too hot inside and the board shut the burners off | The filter, closed vents, furniture over the returns. A dirty blower wheel does it too |
| Ignition lockout | Three tries to light failed, so it locks out and tries again in about an hour | That the gas valve at the furnace is on. A dirty flame sensor or a cracked igniter is the usual cause |
| Flame with no call for heat | The board sees flame when it shouldn’t, often a stuck gas valve | Nothing. Turn the furnace off at its switch and call |
| Polarity or ground | Hot and neutral reversed, or a bad ground | Usually after electrical work. The wiring gets fixed, not the furnace |
The dirty flame sensor is the one we find most in the first cold week. Here’s what a flame sensor looks like and why it quits. If a limit code keeps tripping with a clean filter, a dirty blower wheel is the next suspect, and a limit code that won’t go away is also why we look at the heat exchanger. On a Trane, 7 red flashes point at the gas valve circuit.
Reset it once, then stop
Turn the furnace off at its switch for 30 seconds, then back on. That clears the lockout and the furnace starts over from the top. If it runs and the code doesn’t come back, you probably had a one-off: a gust that blew the flame out, a power blip. Write the code down anyway.
If the same code comes back, stop resetting. Each reset is another round of a furnace trying to start with a fault, and on a limit code that’s another round of overheating. Change the filter if it’s dirty, open the vents, and call. If it won’t even try, our post on a furnace that won’t turn on walks the checks in order. On a Goodman that shuts off after a few minutes or blows cold air, those posts do the same.
Turn it off and call right away if
- You smell gas. Leave the house and call the gas company or 911 from outside.
- The carbon monoxide alarm is going off. Get everyone out first. Here’s what to do when the CO alarm sounds.
- A limit code keeps coming back after you’ve changed the filter.
- The board shows flame with no call for heat, or you see flame with the thermostat off.
- Anything near the furnace is scorched or smells like burning wire.
Stop the code before it starts
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade, which is the fix when a caked wheel is what keeps tripping the limit. We check the flame sensor, igniter, pressure switch and venting on every one. More on heating repair.
Book it
A repair visit starts with a flat $99 diagnostic, or $250 after hours, and you get the price of the fix before we touch anything. Call (703) 997-9757, day or night, or book online from goswiftpro.com/cookiesoffer by November 30 and a box of warm MOLTN cookies comes with the heating check. Every current deal is on our offers page. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
Why is my furnace blinking?
The control board is reporting its status. A steady light or a slow blink is normal. A pattern of quick flashes with a pause is a fault code you match to the chart on the inside of the furnace door.
Is a blinking green light on a furnace normal?
Usually, yes. On many boards a slow green blink means power and no faults, and a fast green blink means the thermostat is calling for heat. Amber, red, or a counted pattern of flashes is a fault.
Does turning the furnace off and on clear the code?
Yes, on most furnaces. Reset it once with the furnace switch. If the same code comes back, stop and call, because the fault is still there.
Should I take the furnace door off to see the light?
No. The door presses a safety switch that cuts power, and on most boards that erases the code. Read the light through the window, take a video, then call.
