Furnace Error Codes: What Those Blinking Lights Mean

When your furnace stops working, the first place to look is the small LED light on the control board. Most modern furnaces use a blinking light pattern to tell you exactly what went wrong. Here's a guide to the most common furnace error codes, what they mean, and whether you need a professional.

How Furnace Error Codes Work

Most furnaces built after 1990 have a small LED light visible through a peephole on the front panel or inside the blower compartment door. The light blinks in a pattern — for example, 3 blinks followed by a pause means "error code 3." Some newer furnaces use two-digit codes with a combination of fast and slow blinks.

Your furnace's owner manual or the label inside the access panel will have a chart matching blink patterns to specific errors. While codes vary somewhat by manufacturer, most follow similar conventions.

Most Common Furnace Error Codes

Code 1: System Lockout (1 Blink)

What it means: The furnace tried to ignite multiple times and failed, so it locked itself out as a safety measure.

Common causes: - Faulty igniter or hot surface ignitor - Gas supply issue (valve closed or low gas pressure) - Dirty or cracked flame sensor

What to do: Turn off the furnace for 30 minutes, then turn it back on. This resets the lockout. If it locks out again, call for service — repeated lockouts indicate a problem that needs professional diagnosis.

Code 2: Pressure Switch Error (2 Blinks)

What it means: The pressure switch, which confirms the draft motor is running and venting properly, is stuck open or not closing.

Common causes: - Blocked exhaust vent or intake pipe (check for ice, debris, or bird nests) - Failed draft inducer motor - Cracked or disconnected pressure switch hose - Condensate drain clog (on high-efficiency furnaces)

What to do: Check your exhaust vent outside for visible blockages. If clear, this usually requires a technician to diagnose the draft motor or pressure switch.

Code 3: Pressure Switch Stuck Closed (3 Blinks)

What it means: The pressure switch is stuck in the closed position, which prevents the furnace from verifying proper venting before ignition.

Common causes: - Failed pressure switch - Short circuit in the wiring

What to do: Call a technician. This is an electrical or component issue that requires professional repair.

Code 4: Open High-Limit Switch (4 Blinks)

What it means: The furnace overheated and the high-limit safety switch tripped to shut it down.

Common causes: - Dirty air filter restricting airflow (the most common cause) - Blocked or closed supply registers - Failed blower motor not moving enough air - Undersized ductwork

What to do: Check your air filter first — a clogged filter is the number one cause of this code. Replace it and reset the furnace. If the code returns with a clean filter, the blower motor or ductwork may need attention.

Code 5: Flame Sensed Without Gas Valve Call (5 Blinks)

What it means: The control board is detecting a flame when there should not be one. This is a safety error.

Common causes: - Faulty flame sensor sending false signals - Gas valve leaking slightly - Control board malfunction

What to do: Turn off the furnace and call for service immediately. This is a safety issue that should not be ignored.

Code 6: Power or Line Voltage Polarity Error (6 Blinks)

What it means: The furnace is detecting incorrect electrical wiring — the hot and neutral wires may be reversed.

Common causes: - Reversed line voltage wiring - Bad ground connection - Faulty outlet or wiring in the home

What to do: This is an electrical issue. Do not attempt to fix it yourself. Call an HVAC technician or electrician to check the wiring.

Code 7: Gas Valve Circuit Error (7 Blinks)

What it means: The control board is unable to properly control the gas valve.

Common causes: - Faulty gas valve - Wiring issue between the control board and gas valve - Failed control board

What to do: Call for professional service. Gas valve issues require a licensed technician.

Code 9: Ignition Failure (9 Blinks)

What it means: The igniter is glowing but the furnace is not detecting a flame after the gas valve opens.

Common causes: - Dirty or failing flame sensor (most common) - Low gas pressure - Faulty gas valve not opening fully

What to do: A dirty flame sensor is the most frequent cause of this code and is one of the most common furnace repairs. A technician can clean or replace it quickly.

Continuous Blinking (No Pause)

What it means: The furnace is operating normally. A steady, continuous blink typically indicates the system is in standby and ready for a heat call.

What to do: Nothing — this is normal operation.

No Light at All

What it means: The control board has no power.

Common causes: - Tripped circuit breaker - Blown fuse on the control board - Failed transformer - Power switch on the furnace turned off

What to do: Check your circuit breaker and the power switch on or near the furnace (it often looks like a light switch). If both are on and there is still no light, the board or transformer may need replacement.

Codes by Manufacturer

While the codes above cover the most common patterns, here are a few manufacturer-specific notes:

Carrier / Bryant: Use two-digit codes displayed on a small LED screen. Code 33 (limit circuit fault) and code 34 (ignition failure) are the most common.

Lennox: Uses a similar blink pattern system. Lennox furnaces also display codes on the SureLight control board. Code 2 (system lockout) and code 4 (high temperature limit) are frequent.

Trane / American Standard: Uses LED blink codes. 2 blinks (system lockout after retries) and 4 blinks (open high-limit device) are the most common.

Rheem / Ruud: Uses blink patterns with a diagnostic LED. 3 blinks (pressure switch) and 4 blinks (high-limit switch) come up most often.

Goodman / Amana: Similar blink code system. Code 4 (open high-limit) and code 9 (ignition failure) are the most frequent service calls.

What You Can Fix Yourself

A few error codes have simple fixes any homeowner can try:

• . Replace the air filter — Fixes code 4 (overheating) in most cases
• . Check the exhaust vent outside — Clear any ice, snow, leaves, or debris blocking the pipe. Fixes code 2 in some cases
• . Reset the furnace — Turn it off for 30 minutes, then back on. Clears lockout codes (1 and 9) temporarily
• . Check the circuit breaker — Fixes "no light" situations
• . Make sure registers are open — Closed vents restrict airflow and cause overheating

When to Call a Professional

Call for service if: - The same code keeps coming back after a reset - You see code 5, 6, or 7 (safety and electrical issues) - Your furnace is over 15 years old and showing frequent codes - You smell gas at any point (leave the house and call your gas company first) - The fix involves gas valves, electrical wiring, or the control board

Save on Furnace Repairs

Energy Solutions maintenance plan members get priority scheduling and repair discounts. Regular tune-ups also catch many of these issues before they trigger error codes.

Call (925) 778-1335 to schedule a furnace repair or ask about our maintenance plans.