A blinking light on your furnace is the control board telling you which safety check stopped the heat. Count the flashes before you touch anything, then match the count to the label on your own furnace. Some codes point to simple fixes like a dirty filter. Others mean you should stop and call.
Safety first: If a CO alarm goes off, leave the home now and call 911 from outside. If you smell gas, leave and, from outside, call 911 and Enbridge Gas's emergency line at 1-866-763-5427. On propane, call 911 and your propane supplier. Don't stop to read codes or reset anything.
What does a blinking light on my furnace mean?
Most modern gas furnaces have a small LED on the control board that flashes in patterns. What each pattern means varies by furnace. On many models, a set number of flashes, then a pause, then the same pattern again signals a fault, and the furnace has shut down or locked itself out to protect you and the equipment. Other patterns can be normal. The code chart inside your own furnace door tells you what your pattern means.
The light can't tell you which part failed. It tells you which safety check tripped, like the airflow limit, the venting pressure switch or the flame sensor.
How do I read the code without erasing it?
Read it through the small window on the lower panel, and write the code down before you open any panel or switch anything off. What happens to the code after that varies by furnace, so note it first.
Steps to note the code
- Find the small viewing window (sight glass) on the lower furnace door.
- Count the flashes between pauses. Watch two full cycles.
- Note the colour, and whether the light is solid, blinking fast or blinking slowly.
- Take a short phone video.
- Note the brand and model if the label is visible without removing panels.
Why can't I just look up the code online?
There's no standard code across brands, or even across models from the same brand. The same number of flashes can point to the venting pressure switch on one furnace and an airflow limit on another. Some newer furnaces show letters and numbers or use two coloured lights instead.
The code chart that applies to you is printed on a label inside your own furnace door, or in its owner's manual. Use that, not a generic chart online.
Which codes can I safely do something about?
Only two situations are safe to handle yourself: no light at all, which usually means no power, and limit or airflow codes, which often come from a dirty filter or blocked vents. Everything else needs someone who works on furnaces. That includes ignition, flame, gas valve, rollout and board codes, so leave those alone and call.
No light at all
This usually means no power. Check that the furnace switch, which often looks like a light switch near the unit, is on. Then check for a tripped breaker. Reset a tripped breaker once. If it trips again, leave it off and call.
Limit or airflow codes
These often come from a clogged filter or blocked vents. Replace a dirty filter, open closed registers, and move furniture off return grilles. Our furnace not working checklist walks through these checks in order.
For any other code, don't open burner compartments, touch wiring or bypass switches.
Can snow cause a furnace error code?
Yes. High-efficiency furnaces vent through white plastic pipes on an outside wall. If snow or ice blocks those pipes, the furnace can shut itself off. Which code it shows varies by furnace; the chart inside your furnace door explains yours. Blocked vents can also let carbon monoxide build up indoors. In the snowbelt north and west of Barrie, and in drifting off Lake Simcoe, check the vents after heavy snow or wind. Our guide to furnace vents blocked by snow shows how to find the pipes and clear them safely.
Clearing a snow-blocked vent safely
- Brush snow away from the outside openings with a soft broom or brush.
- Don't scrape with sharp tools, kick the pipes, or blow snow against them.
- Never take vent pipes apart.
- If the furnace still won't run after the vents are clear, call.
Is it safe to reset the furnace when the light is flashing?
Note the code first. After that, one reset is reasonable for a code you understand, like a limit trip right after you replaced a clogged filter. If the same code comes back, leave the furnace off and call. Resetting over and over can hide what's wrong or make it worse.
Oil furnaces: press the reset button once at most
Some rural homes in Oro-Medonte and Springwater heat with oil or propane. On an oil furnace, press the reset button on the burner control once at most. Manufacturers warn that pressing it again and again can pump unburned oil into the combustion chamber, which can cause a fire or puff-back. If the burner doesn't start after one press, turn it off and call.
When does a flashing light mean I should call right away?
Call now if the code keeps coming back, the house is getting cold, or the code points to ignition, flame, venting, gas valve or rollout. In a Barrie cold snap, a lockout overnight can quickly lead to cold rooms and frozen pipes. Furnace Repair Barrie answers 24/7 at (249) 209-2216.
Our emergency furnace repair page explains what to expect when you call. If the furnace quit after dark, see what to do when your furnace won't turn on at night.
If the code shows up along with a burning, sulphur or rotten-egg smell, follow the safety steps in our furnace smells like gas or burning guide before anything else.
What should I have ready when I call?
Have the flash count and colour (or your video), the furnace brand and model, and when you last changed the filter. Mention any resets you tried, whether the house is getting colder, any smells, and your town. Call Furnace Repair Barrie 24/7 at (249) 209-2216.