What does the blinking light on my furnace mean?
The blinking light is the status LED on the furnace control board, and the number or pattern of flashes is a fault code. Match it to the chart on the inside of your furnace door. Most codes point to a pressure switch, flame sensor, limit switch or ignition lockout; a steady light usually means normal.
Key points
- The light is the control board's status LED; the flash pattern is a code, and the chart on your furnace door translates it.
- Brands count differently: Goodman uses single flash counts, Carrier uses two-digit codes built from short and long flashes.
- Read it through the sight glass with the door on; pulling the door usually cuts power and can clear the code.
- One reset is reasonable for an ignition or pressure switch lockout after you clear the vents; a limit or rollout code means stop and call.
- Write down the code, the brand and the model number before you call; it shortens the repair.
The blinking light on your furnace is the status LED on its control board, and the number or pattern of flashes is a fault code telling you what the furnace thinks went wrong. Count the flashes and match them to the code chart printed inside the furnace door, because each brand uses its own chart and the same count can mean different things on different furnaces.
A steady light, or a slow even blink while it's heating, is usually normal. A pattern that repeats with a pause, such as three flashes, a pause, three flashes, is a fault. The code won't always name the broken part, but it tells you and the technician where to start.
Where is the furnace status light, and how do you read it safely?
On most gas furnaces the control board sits behind the lower door, the blower compartment. Many doors have a small sight glass so you can see the LED without removing anything.
- Leave the door on. Most furnaces have a door switch that cuts power when the blower door comes off, and when power comes back the code you wanted may be gone. Trane's troubleshooting page for blinking furnace lights says plainly not to take the front panel off while the furnace is running.
- Watch for at least 30 seconds. Carrier's troubleshooting guide for its 59MN7A modulating furnace tells technicians to watch that long to be sure a slow pattern of short and long flashes isn't being mistaken for something else.
- Count and write it down. Note the number of flashes, the color, and whether the pattern repeats.
- Photograph the chart and the rating plate. The chart is usually on the back of the door. The model number is on the rating plate inside the cabinet.
How do furnace brands count the flashes?
Some furnaces use a single count. Goodman's installation instructions for its single-stage condensing furnaces list this chart:
| Flashes | Goodman meaning (single-stage chart) |
|---|---|
| 1 | System lockout |
| 2 | Pressure switch stuck closed |
| 3 | Pressure switch stuck open |
| 4 | Open limit switch |
| 5 | Flame sensed when no flame should be present |
| 6 | Open rollout switch |
| 7 | Low flame sense signal |
| 8 | Igniter relay fault |
| Rapid, continuous | 115-volt line power polarity reversed |
| Steady on | Normal operation |
| Off | Control failure |
Others use two-digit codes. Carrier's 59MN7A guide explains that the first digit is the number of short flashes and the second is the number of long flashes. On that furnace, 13 is a limit circuit lockout, 14 is an ignition lockout, 32 means the low pressure switch didn't close, 33 is a limit circuit fault and 34 is an ignition-proving failure. Lennox, Trane, Rheem and others have their own charts again. Use this table to understand the families, then trust the label on your furnace.
What do the common furnace error codes mean?
Pressure switch. The furnace couldn't prove the inducer is venting exhaust. In a Minnesota winter that is usually snow or frost over the PVC vents, a frozen or plugged condensate drain, or a weak inducer. Carrier's guide lists a restricted vent, a plugged condensate drain, water in the vent pipe and excessive wind among the causes. Our post on the furnace pressure switch goes through it in order.
Flame sense. The burners lit but the board couldn't confirm the flame, so it shut the gas off. A dirty flame sensor is the usual cause, and a poor ground or low gas pressure are others. Cleaning the sensor is part of a normal tune-up, and our flame sensor post explains what it does.
Limit. The furnace got too hot and the limit switch shut the burners off. That almost always means too little airflow: a clogged filter, closed registers, a blocked return or a weak blower. See what a furnace limit switch does.
Rollout. Flame came out of the burner area instead of being drawn into the heat exchanger. Goodman's instructions describe its rollout switches as manual-reset safeties that guard against exactly that. This is not a homeowner reset. It can point to a blocked heat exchanger or vent.
Ignition lockout. The furnace tried to light several times and gave up. Goodman's instructions define lockout as failing to achieve ignition after three attempts in one call for heat, and say the board resets itself after one hour. Carrier's 59MN7A guide says its ignition lockout resets automatically after three hours. Trane describes the first one or two failed tries as a soft lockout and a third as a hard lockout. Our furnace won't ignite guide walks through where the startup sequence stops.
What to check before you call
- Thermostat. It's set to Heat, the setpoint is a few degrees above room temperature, and the screen is on. Replace the batteries if it uses them.
- Breaker and furnace switch. The furnace switch (it looks like a light switch on or near the furnace) is on, and the breaker hasn't tripped. The blower door is pushed tight against its switch.
- Filter. A clogged filter is the most common cause of limit codes. Change it if it's gray.
- Outside vents clear. On a high-efficiency furnace, clear snow and frost from the PVC intake and exhaust by hand.
- Condensate. Look for water around the furnace, and listen for gurgling at startup, which can mean a frozen or plugged drain line.
Then reset once. Turn the furnace switch off for a minute and back on, and watch the next cycle. Goodman's instructions note that if the condition that caused the lockout is still there, the control will go right back into lockout. That's your answer: stop resetting and call.
Never reset a furnace over and over, and never bypass a pressure, limit or rollout switch to get heat. Those switches exist to keep exhaust and flame where they belong. If you smell gas, leave the house and call the gas utility or 911 from outside.
What does a technician do with a furnace error code?
The code is where we start, not where we stop. We read it with the door on, pull the stored fault history if the board keeps one, then test the part the code points to: suction at the pressure switch with a manometer, flame signal in microamps, temperature rise and static pressure for a limit fault, and the vent and drain for anything draft-related. Carrier's guide, for example, gives a minimum flame signal and a normal range for its sensor, which is the kind of number we check instead of guessing.
Nationally, the parts behind these codes are modest. HomeGuide puts a typical national range at $150–$400 for a pressure switch, $80–$250 for a flame sensor replacement, $100–$375 for a limit switch and $300–$650 for a control board. With us, a furnace repair visit is a $79 trip charge, waived when you approve the repair, with a written, flat-rate price before any work starts. Our furnace repair cost guide covers the rest.
When to stop and call
Call if the same code comes back after one reset, if the code is a limit, rollout or flame-sensed-with-no-call fault, if the light is off with the power on, or if the furnace is short-cycling and shows a different code each time. Tell us the brand, the model number and exactly what the light did. We're open 7 days, 7am–9pm, and no-heat calls get priority, including furnace repair in Minneapolis and Saint Paul. Call 612-429-3465.
Questions
What does a steady, solid light on my furnace mean?
On most furnaces a steady light means the control board has power and no fault is active. Goodman's installation instructions for its single-stage furnaces list a steady light as normal operation, and Carrier's troubleshooting guide for its 59MN7A furnace says continuous on means the control has 24-volt power. If the light is steady and you still have no heat, look at the thermostat and the vents outside next.
What does it mean if the furnace light is off?
No light at all usually means the board has no power. Check the furnace switch near the unit, the breaker, and that the blower door is seated firmly on its switch. If all of that is fine and the light stays dark, the board, its transformer or a low-voltage fuse may have failed. Goodman's chart lists an off light as a control failure, which is a job for a technician.
My furnace light is flashing very fast without stopping. What is that?
On both the Goodman and Carrier charts we read, a rapid, continuous flash means the 115-volt line power to the furnace has its polarity reversed, which usually happens after electrical work or a new furnace install. The burners may not light or may drop out because the board can't read the flame properly. It's a wiring fix for a technician or electrician, not something to reset.
Can I clear the fault code history on my furnace?
Some boards store past codes and have a recall or reset button, but clearing them doesn't fix anything, and the history helps a technician see how often a fault has happened. Leave it alone and write down what you see. If the current fault is still there, the code will come back on the next call for heat.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com