How to Use the Fault Code Lookup
Select the category that best matches what your equipment's fault indicator is showing, add a bit of context, and get plain-language guidance on what that category of fault typically means and what to check next.
Every manufacturer designs its own proprietary blink code logic, a 3-blink code means something entirely different on a Carrier versus a Trane, Lennox, Goodman, or York board, and even varies between control board generations from the same manufacturer. Reproducing manufacturer-specific tables would be both inaccurate and risk misrepresenting proprietary documentation, so this tool focuses on the diagnostic categories that are genuinely universal across the industry.
The decoder sticker inside the blower access panel or control board cover is the definitive source for your equipment's exact fault code meaning. If missing or worn, search for the installation and service manual using the model number on the equipment's data plate. See the furnace not heating tool for diagnosis beyond code categories.
Understanding HVAC Fault Code Categories Across Manufacturers
Modern HVAC control boards use LED indicators, typically a single light that blinks in specific patterns, to communicate diagnostic information about why the equipment has stopped or refused to start. While the exact blink patterns are manufacturer-proprietary, the underlying fault categories they represent are remarkably consistent across the industry, since they reflect the same basic set of safety and operational checks that virtually all modern gas and electric HVAC control systems perform.
Ignition Lockout: The Most Common Furnace Fault
Ignition lockout occurs when a gas furnace's control board attempts to establish a flame (through hot surface ignition, intermittent pilot, or direct spark ignition depending on the system) but fails to prove a stable flame within the allowed number of attempts, typically 3-4 tries. The control board then "locks out," refusing further ignition attempts until manually reset, as a safety measure to prevent continuously venting unburned gas into the heat exchanger and flue. Common underlying causes include a weak or aging ignitor no longer reaching adequate temperature, a fouled or misaligned flame sensor unable to detect the flame that IS present, low gas supply pressure, or a gas valve not opening fully. See the furnace not heating tool for a broader furnace fault analysis.
Pressure Switch Faults: Venting-Related Issues
Modern condensing and induced-draft furnaces use a pressure switch to confirm the inducer motor is running and creating adequate draft (negative pressure) in the venting system before allowing gas ignition -- a critical safety check ensuring combustion byproducts can properly vent. A pressure switch fault indicates this switch didn't close (or didn't open) at the expected point in the startup or shutdown sequence. This can stem from a failing inducer motor, a blocked or restricted vent pipe, a blocked condensate drain causing water to back up into the pressure switch tubing on condensing furnaces, or a failed pressure switch itself. See the pressure switch calculator for pressure-specific analysis and the flue gas analyzer for venting performance.
Flame Sensing Faults
After successful ignition, the control board continuously monitors a flame sensor rod to confirm the flame remains stable throughout the burn cycle -- if the sensor loses its signal, the board shuts off gas flow immediately as a safety measure, since burning gas without a monitored flame is a hazardous condition. Flame sensing faults, distinct from ignition lockout faults, indicate the flame was initially established but then lost, or that the sensor itself isn't reading a flame that's actually present due to fouling, misalignment, or a degraded sensor rod requiring cleaning or replacement.
Limit Switch Trips: Overheating and Overpressure Protection
Limit switches are safety devices that open (interrupting operation) when temperature or pressure exceeds safe operating thresholds -- a high limit switch protects against heat exchanger overheating from restricted airflow, while a high-pressure or low-pressure switch on refrigerant systems protects the compressor from operating outside safe pressure ranges. A limit switch trip fault code points toward an underlying condition causing the monitored parameter to exceed its threshold, most commonly restricted airflow in the case of high-temperature limit trips. See the airflow diagnosis calculator for airflow-related limit trip causes.
Communication and Control Board Faults
Modern HVAC systems, particularly those with variable-speed or communicating equipment, often use multiple control boards (an integrated furnace control, a variable-speed blower control, an outdoor unit control, and a communicating thermostat) that exchange data over an internal communication bus. Communication faults indicate a break in this expected data exchange -- possibly from a loose or damaged wiring connection, a failed control board, or a communicating thermostat that's lost its connection to the system. These faults are increasingly common as HVAC systems adopt more sophisticated, interconnected control architectures, and typically require more specialized diagnostic tools than a simple blink code to fully resolve.
Frequently Asked Questions
Each manufacturer designs its own control board logic and assigns its own specific blink patterns or codes to represent fault conditions. There's no industry-wide standard blink code system, so the same blink count can mean completely different things across manufacturers. This is why looking up an exact code always requires the specific manufacturer's documentation -- ideally the decoder sticker inside the access panel or the service manual for that exact model.
Most furnaces have a fault code decoder sticker on the inside of the blower access panel or control board cover, listing that model's specific blink patterns and meanings. If missing or worn, the installation and service manual (available from the manufacturer's website using the model number from the data plate) contains the complete reference. Be cautious of generic "universal" code lists online, since a code number alone is meaningless without confirming it matches your specific manufacturer and control board revision.
Across virtually all manufacturers: ignition lockout (failed to establish/prove flame within allowed attempts), pressure switch faults (didn't close/open at expected sequence point, often venting-related), flame sensing faults (flame not detected after ignition), limit switch trips (high temperature/pressure safety limit opened), and communication faults (signal not received as expected between control boards). Understanding the category helps narrow the diagnostic direction even before consulting manufacturer-specific documentation.
Repeatedly resetting a furnace without addressing the underlying cause is not recommended. A single reset can sometimes clear a transient fault, but repeated resets on a persistent fault -- especially ignition lockouts or limit switch trips -- can indicate an underlying safety-related condition that won't resolve itself and may worsen. If a fault recurs after one reset attempt, note the specific blink pattern and contact a licensed technician rather than continuing to cycle power.