Quick answer
A flashing light on your furnace is a diagnostic code, not a warning lamp. Count the flashes and the pauses between them, then read the code legend printed inside the blower door. The pattern maps to a specific fault — an open pressure switch, an open limit, a failed ignition attempt, or flame sensed when there should be none. Numbering differs by manufacturer.
The small window in the lower door of a furnace shows a status light, and when it flashes it is reporting a fault code the control board has stored. Nearly every furnace built in the last three decades does this, and nearly every one carries a legend for those codes on a sticker inside the blower compartment or on the back of the door. That sticker is the authority. Flash patterns are not standardized between manufacturers, so a code that means one thing on a Goodman means something else on a Carrier.
How to read a furnace flash code
- Watch through the sight glass without opening the door. Opening it trips the door switch, cuts power to the board, and can wipe the stored code.
- Time the rhythm. A long pause, a group of flashes, another long pause — that is one code repeating. Count the flashes inside a single group.
- Watch for two groups. Many boards use a two-part code: three flashes, a short pause, two flashes. That reads as 3-2, not as five.
- Note the colour and the speed. Some boards use green for normal and red or amber for faults; some use a steady light for normal and a slow once-per-second heartbeat to say the board is powered and idle.
- Record it on your phone. A five-second video of the pattern is the single most useful thing you can hand a technician on the doorstep.
What the common fault families mean
Once you have a number, the sticker turns it into a sentence, and those sentences fall into a handful of families:
- Open pressure switch — the board never received proof of draft from the inducer.
- Open limit — the furnace overheated, usually behind a restricted filter or a closed-up duct system.
- Ignition lockout — the burners were given three or four chances and never produced a flame the board could read.
- Flame present with the gas valve closed — the board is seeing a signal it should not, which points at the sensor circuit or a valve that is not seating.
Two more show up often and confuse people. A rollout or auxiliary limit code refers to a manual-reset safety near the burners, and it should never be cleared a second time without a cause being found. A polarity or grounding code means the furnace is wired hot-to-neutral backwards or has lost its ground path — flame rectification cannot work without a solid ground, so the burners light and immediately quit.
Look up the code for your brand
Because the numbering is brand-specific, the fastest route is the manufacturer's own list. We keep code guides for the furnaces we see most across Halton and Hamilton: Carrier, Lennox and Goodman. Newer premium boards skip flash patterns altogether and show a two-character alphanumeric code on a small display, which is easier to read and just as brand-specific.
What the code does not tell you
A code names the switch that opened. It does not name the reason it opened. A pressure switch fault is equally correct in a house where a February drift buried the sidewall vent and in one where a condensate trap has filled with sludge — the board cannot tell those apart. That is why a technician still tests rather than reads: a manometer on the inducer port, eyes on the vent termination, a current reading on the motor.
It also does not say how often it has happened. Some boards keep a short history of recent faults, and that history is worth more than a single live code. An intermittent limit trip logged 11 times since November is a different conversation from one that happened once during an ice storm.
Codes that mean stop using the furnace
- Any rollout code, anything the sticker describes as a flame or gas-valve fault, and anything at all accompanied by an odour — see why a furnace smells like gas.
- A limit code that returns every cycle after you have already changed the filter. Something structural is choking the airflow.
- A code showing at the same time as a carbon monoxide alarm. Get everyone outside first and make the call from there.
- No light at all on a furnace that used to have one, with a dead blower. That is usually the low-voltage fuse on the board or the transformer feeding it, not the board itself.
Most of what these codes point to is inexpensive: the majority of fixes land in the $150–$450 band, on top of a $90–$150 diagnostic that we credit toward the work. Give us the flash pattern and the brand when you call (647) 801-1252 and we can usually load the right part before leaving the shop. Furnace repair covers Burlington, Oakville, Hamilton and the wider GTA — book a visit here.