Furnace Blinking Light: How to Read Flash Codes

Most gas furnaces built in the last few decades have a self-diagnostic control board with an LED. When something goes wrong, the board shuts the furnace down safely and blinks a pattern that names the fault. Learning to read it takes two minutes, and it can turn a vague "no heat" call into a specific fix, or tell you the problem is a snowdrift outside.

Common code families and what to do

Wording and numbers vary, but nearly every board reports the same handful of faults. Once you have read your code (steps below), this is what it usually means.

How to read the light, step by step

1. Find the window. Look at the lower (blower) door for a small clear lens. The LED may be red, green, amber, or a pair of them.
2. Watch a full cycle. Codes repeat with a pause between. Count the flashes between pauses at least twice.
3. Film it. A 30-second phone video settles any counting doubts and is useful to a technician.
4. Note the style. Is it one length of flash, or long and short flashes? Is there a digital readout instead?
5. Find the chart. It's usually on a label inside the blower door or on the back of the upper door. Turn off the furnace's service switch before you remove a door to read it, and only after you've recorded the code. The door switch cuts power when the panel comes off.
6. Match and decide. Use the table below to judge whether it's a homeowner check or a service call.

The four ways furnaces flash codes

Simple flash count

The LED blinks a number, pauses, and repeats. Goodman, Amana and Daikin furnaces generally work this way, as do many Trane, American Standard and York models. A steady or slow, regular blink usually means normal operation. Goodman's GKS9 service manual, for example, shows no light when the board has no power, and 1 flash for a lockout after repeated failed ignition.

Long and short flashes (two-digit codes)

Carrier and Bryant boards use a number of long flashes for the first digit and short flashes for the second, so "3 long, 3 short" means code 33. Don't read it as six flashes.

Digital display

Some current Lennox furnaces show "E" codes such as E 223 on a small display, and Rheem's 802PA series shows two-digit codes on a seven-segment readout. The code may alternate with a status reading.

Two LEDs

Some York models use a green status light and a separate red fault light. Green tells you the furnace's state; red tells you what went wrong.

About lockouts and resets

After several failed tries, most boards go into a lockout and many retry on their own after a set time. Goodman's GKS9 manual, for example, says the control tries to reset itself after one hour. You can clear a lockout sooner by switching the furnace off at its service switch, waiting half a minute, and switching it on. Do that once, after you've recorded the code and fixed anything obvious. If the same code returns, call.

Brand-specific code charts

We keep code lists, warranty notes and Colorado altitude tips for the brands we service most:

• Lennox: digital E-codes on many current models
• Carrier and Bryant: long-and-short two-digit codes
• Trane and American Standard: red LED flash counts
• Goodman, Amana and Daikin: flash counts on a shared board family
• Rheem: two-digit display codes on many models
• York: green status and red fault LEDs

Your door label still wins if it disagrees with any list, ours included. Older and newer boards from the same company can use different numbers for the same fault.

Colorado codes we see most

• Pressure switch codes after snow. Drifts over sidewall PVC vents and frost inside them are the classic Front Range cause. The DOE's high-efficiency furnace guideline calls for terminations a foot above the expected snow level. Clear them by hand; never cover them.
• Pressure switch codes during deep cold. Condensate lines through garages and attics freeze and back water into the trap.
• Limit codes in fall. Filters loaded with summer wildfire smoke and dust trip the limit early in the season.
• Ignition and flame codes at altitude. Thinner air leaves less margin. A furnace not set up for elevation, or with a dirty flame sensor, is more likely to fail ignition here. Our altitude guide explains why.
• Board and fuse faults after storms. Lightning strikes and power blips sometimes kill a circuit board or blow its low-voltage fuse.

When to call

• Any rollout, flame-with-gas-off, polarity, or motor code.
• Any code that returns after one reset.
• A pressure switch code with clear vents and a dry drain.
• No light and no power after checking the switch and breaker.

When you phone MrFixer Heating & Cooling, (720) 466-7983, give us the brand, the model number from the door label, and the code. It helps us bring the right parts. We do furnace repair on all major brands across the Denver metro, with emergency service when it can't wait. If the furnace won't start at all, start with our furnace won't turn on guide; for flame-sensor-style on-off cycling, see furnace short cycling. Wondering whether a board replacement is worth it on an older unit? Our furnace repair cost guide covers repair-or-replace.

Frequently asked questions

Why is the light on my furnace blinking?

The control board is reporting its status. A steady or slow, regular blink usually means normal operation. A counted pattern with a pause means a fault. Compare it with the legend printed on the furnace door panel.

How do I read Carrier's long and short flashes?

Long flashes give the first digit and short flashes the second. Three long followed by three short is code 33, not six. See our Carrier and Bryant pages for code meanings.

Will turning the furnace off clear the error code?

Usually, yes. Cutting power or opening the blower door generally stops the flashing and may erase the last code, so record it first.

What does a pressure switch code mean on a high-efficiency furnace?

The furnace can't confirm a clear vent. In Colorado the most common cause is snow or ice blocking the outdoor PVC pipes, followed by a frozen or plugged condensate drain. Clear the pipes by hand; call if the code returns.

Is it safe to keep resetting a furnace that shows a code?

A single reset, once you've dealt with anything obvious, is reasonable. Repeated resets on ignition, rollout or flame codes can let gas build up or mask a cracked heat exchanger, so switch it off and get it looked at.

Where is the furnace code chart?

Usually on a label inside the blower door or the upper burner door. Turn off the service switch before removing a door, and only after you've recorded the code. If the label is missing, the installation manual for your model has it.