HVAC Diagnostics · Optical Pulse Simulator
Furnace Blinking Light &
Flash Code Decoder
Identify Carrier, Goodman, Lennox, Trane, and York blinking LED status patterns. Watch the interactive virtual LED pulse simulator, match blink tempo, and review manufacturer-verified safety stop conditions.
Diagnostic optical decoder
Decode LED blink patterns through the inspection viewport.
Modern residential gas furnaces communicate operational status and safety lockouts through flashing red, amber, or green LEDs on the integrated furnace control board. Observe the light through the blower door viewing glass (do not remove access panels, which cuts power and erases the active fault code) and select your brand or tap the tempo below.
Step 1: Select Brand & Observed Pattern
Step 2 (Optional): Tap Blink Tempo
Tap the button in rhythm with your furnace light to measure cadence and auto-detect patterns.
Observe the blinking LED through the glass and tap with each flash...
Virtual LED Simulator
Simulated optical timing & pulse cadence
Tip: Carrier displays use rapid flickers or two-digit pauses (e.g. 3 flashes, short pause, 1 flash = Code 31). Lennox uses dual red/green LEDs. Goodman, Trane, and York rely on single-pulse repeating counts.
Code 31
Venting & Draft Fault: Inducer pressure switch did not detect adequate venting vacuum.
Draft safeguard or pressure switch did not close or reopened during the combustion cycle.
Inducer motor runs but burners never ignite, or burners shut off shortly after lighting.
- Inspect the PVC exhaust and intake vent pipes outside your home to ensure they are free of snow, ice, or nests.
- Inspect the condensate drain tubing visible outside the cabinet for kinks, freezing, or water backups.
- Verify that the furnace filter is seated properly and not causing extreme static pressure.
- Do not disconnect or blow directly into pressure switch silicone hoses.
- Do not operate the furnace if vent pipes are disconnected, damaged, or leaking combustion exhaust indoors.
- If vents are clear outside, shut down the furnace and contact a licensed HVAC professional to inspect the inducer assembly.
Static OEM Charts
Furnace Flash Code Reference by Manufacturer
Review documented diagnostic flash patterns for top residential furnace makers. All codes are verified against primary manufacturer installation and service documentation.
Carrier Furnaces
9 Codes- Rapid Flash115V AC line voltage polarity reversed or twinning circuit connection fault...
- Code 12Blower on after power-up; 90-second blower delay running following power re...
- Code 13Limit switch circuit lockout; primary limit switch has remained open longer...
- Code 14Ignition lockout; control board attempted maximum ignition trials without p...
- Code 31Draft safeguard or pressure switch did not close or reopened during the com...
Goodman Furnaces
8 Codes- 1 FlashIgnition lockout; furnace failed to ignite or sense flame after 3 consecuti...
- 2 FlashesDraft inducer pressure switch stuck in the closed position before inducer f...
- 3 FlashesPressure switch failed to close or reopened during the draft inducer fan ru...
- 4 FlashesPrimary high-limit switch open; heat exchanger temperature exceeded maximum...
- 6 FlashesFlame sensed with gas valve de-energized; flame signal detected when burner...
Lennox Furnaces
7 Codes- Slow Simultaneous FlashNormal operating mode with no call for heat; furnace is in standby ready st...
- Fast Simultaneous FlashNormal active heating operation; furnace is firing and delivering heat....
- Alternating Red / Green Flash115V AC power polarity reversed or improper electrical earth ground connect...
- 1 FlashFlame sensed without gas valve energized; unproven flame signal present whi...
- 2 FlashesCombustion air pressure switch open or pressure switch did not close within...
Trane Furnaces
8 Codes- Continuous Slow FlashNormal standby operation; integrated furnace control board is energized wit...
- Continuous Fast FlashNormal heating call mode; call for heat initiated and burners firing normal...
- 2 FlashesExternal lockout; ignition failure after allowable retries or flame recycle...
- 3 FlashesDraft inducer pressure switch error; switch failed to close or opened durin...
- 4 FlashesThermal protection device open; primary high-limit switch or thermal rollou...
York Furnaces
9 Codes- Continuous Slow FlashNormal standby operation; furnace control board is powered and awaiting the...
- 1 FlashIgnition lockout; system failed to ignite after maximum trial attempts and ...
- 2 FlashesPressure switch stuck closed prior to draft inducer motor start....
- 3 FlashesPressure switch failed to close or opened during run cycle....
- 4 FlashesPrimary high-limit switch open due to excessive heat exchanger temperature....
Frequently Asked Questions
What does 3 red flashes mean on a Goodman furnace?
On Goodman gas furnaces, 3 red flashes indicates that the combustion air draft pressure switch failed to close or reopened during the draft inducer fan run cycle. Common causes include a blocked exterior PVC exhaust or intake vent pipe (such as snow, ice, or animal nesting), a kinked or clogged condensate drain line, or a clogged air filter creating excessive static restriction. If exterior pipes and drains are unblocked, an HVAC technician must test the draft inducer vacuum pressure using a digital manometer.
Why is my Carrier furnace blinking code 31?
Carrier furnaces communicate diagnostic codes through dual-flash sequences, where Code 31 consists of 3 short flashes followed by 1 long flash (or 3 primary pulses and 1 secondary pulse). Code 31 indicates that the draft safeguard or pressure switch failed to close within the allowable startup interval, or opened during the heating cycle. Safe homeowner checks include inspecting exterior vent terminals for snow/ice blockages and verifying that the condensate drain line is draining smoothly without kinks or freezing.
How do I count furnace blinking lights accurately?
Observe the diagnostic LED through the small clear sight glass on the furnace blower compartment door without opening the panels, as the door safety interlock switch will immediately cut power and wipe the active diagnostic memory. Watch the light complete a full sequence, note the color (red, amber, or green), count the individual flashes until a distinct 1.5 to 3 second pause occurs, and count 2 to 3 repeating cycles to ensure accuracy.
What does a rapid blinking furnace light mean?
On most major furnace manufacturers including Carrier, Goodman, and Lennox, a continuous rapid flash (flickering rapidly multiple times per second without pausing) signals that the incoming 115V AC line voltage polarity is reversed or the electrical earth ground connection is missing. This prevents the flame rectification circuit from establishing safety ground and creates an electrical hazard. Turn off the furnace power switch and have a licensed electrician or HVAC technician verify line voltage polarity and chassis bonding.
Can I reset a furnace by turning the power switch off and on?
Switching off the furnace electrical service switch for 60 seconds executes a power-cycle reboot that clears soft lockouts (such as temporary ignition retries after interrupted gas delivery). However, if an underlying mechanical or safety failure is present—such as an open high-limit switch, blocked exhaust flue, or dirty flame sensor—the control board will re-display the flash code as soon as the sequence fails again. Never bypass safety switches or repeatedly reset a furnace experiencing thermal rollout or gas valve faults.
Safety & Compliance Disclaimer: This interactive furnace blinking light decoder is an informational reference compiled from published manufacturer installation instructions, service manuals, ANSI Z21.47 (Gas-Fired Central Furnaces), NFPA 54 (National Fuel Gas Code), and NFPA 70 (National Electrical Code). It does not constitute professional on-site diagnostic service or repair instruction. If you detect unburned natural gas odors, scorched wiring smells, carbon monoxide alarm activation, or flame rollout, immediately evacuate the structure, shut off the gas supply if accessible, and contact 911 or your local gas utility.
Verified Primary Sources: Carrier Residential Heating (carrier.com), Goodman Manufacturing (goodmanmfg.com), Lennox Residential (lennox.com), Trane Residential Resources (trane.com), and York Heating (york.com).