How do I read my furnace error codes?

Most modern furnaces communicate problems through a blinking LED light on the control board, typically visible through a small sight glass or panel on the furnace cabinet. The number of blinks in a sequence corresponds to a specific error code listed on a diagnostic chart — usually found on a sticker inside the furnace door.

How to read the blink pattern: The LED will flash a series of blinks, pause, then repeat. For example, 3 blinks, a pause, then 3 blinks again means "Error Code 3." Some furnaces use a two-digit system — 3 blinks followed by 2 blinks would be Code 32. Count carefully and write down the pattern before opening any panels or touching anything.

Finding your error code chart: Once you have the blink count, look for the diagnostic sticker on the inside of your furnace's lower access panel. It will list each code and its meaning (e.g., "Code 3 = Pressure Switch Fault"). If the sticker is missing or faded, search your furnace's model number online — you can usually find the full diagnostic chart in the manufacturer's installation manual.

Common codes Ottawa homeowners see include pressure switch faults (often caused by a blocked condensate drain on high-efficiency furnaces), ignition failures, and flame sensor issues. These are all repairable, but the underlying cause matters — a pressure switch fault could mean anything from a dirty filter to a cracked inducer motor.

On the safety side, if your furnace is throwing error codes related to the heat exchanger, flame rollout, or high limit switch, stop running the system and call a technician immediately. These can indicate conditions that allow carbon monoxide to enter your home. Never ignore a heat exchanger warning.

Once you have your error code, a licensed technician can diagnose the root cause quickly. If you'd like help interpreting a specific code, feel free to share it here — or contact Ottawa HVAC Pro for a diagnostic visit, typically $200–$350, which is applied toward the repair cost.