What do the blinking lights on my furnace mean? Reading the codes

The control board runs the heating sequence and watches the safety switches. When one trips, the board shuts the gas off and flashes a code so a technician can see what happened without opening the furnace. Each brand has its own code system. Codes often appear for the first time on the first cold nights of fall, when a furnace that sat idle all summer has to run hard.

Where is the status light on my furnace?

The light sits on the control board behind the lower door, the blower door, and you read it through a small clear window in that door, called a sight glass or viewing port. Some newer furnaces show a number on a small digital display instead. The code chart is on a service label on one of the doors, often next to the wiring diagram. If it is missing, the model number on the rating plate lets you download the installation manual, which has the same table.

Can I take the furnace door off to see the light?

No. Read it through the window with the door on. The blower door holds a door switch closed; removing the door cuts power to the furnace, and on some boards clears the code. Holding or taping that switch in puts your hands near live 120-volt wiring and moving parts, and an open blower door can let the blower draw combustion gases into the house. Manufacturers warn never to defeat it.

How do I read the flash code?

Record the light for a minute with your phone and count from the video. Most boards flash the code, pause, and repeat it.

1. Note the colour and whether there is one light or two.
2. On single-digit boards, count the flashes between pauses: three flashes, pause, three flashes is code 3.
3. On Carrier, Bryant and KeepRite (ICP) boards, short flashes give the first digit and long flashes the second, so two short and one long is code 21, not 3.
4. On a two-light Lennox board, the meaning is the pair: note what each light is doing.
5. Write the code down before you touch the power, because the technician will ask for it.

Steady patterns differ by brand too:

• Slow blink: on Lennox two-light boards and Trane green lights, power on and no call for heat. Normal.
• Fast blink: normal during a heat call on those boards. On Carrier, Bryant and Goodman boards, constant rapid flashing means reversed 120-volt polarity or poor grounding, a wiring fault.
• Steady on: normal on Goodman, power present on Carrier and Bryant. Both Lennox lights steady on means a board or wiring fault.
• Off: usually no power to the board, a blown fuse on the board or a failed board. Check the furnace switch and breaker first.

What do common furnace codes mean by brand?

These codes come from manufacturer manuals for one furnace series per brand. Boards change over the years, and the same number means different things across brands: 33 is a limit fault on a Carrier but a tripped rollout switch on a Rheem. Treat them as examples and go by your own label.

What does each type of fault usually mean?

• Pressure switch: the inducer fan could not prove a clear vent. Usual causes are snow, ice, leaves or a nest in the outside pipes, a plugged condensate drain, water in a sagging vent pipe, or a cracked switch hose.
• Limit: the furnace overheated and the high-limit switch shut the burners off, most often from a clogged filter, closed registers or blocked returns.
• Rollout: flame came out of the burner area, which points to a blocked heat exchanger or vent and is a carbon monoxide risk. Only a technician should reset it.
• Ignition or flame: the burners did not light or the flame was not sensed, usually a dirty flame sensor, a worn igniter, low gas pressure or a gas valve fault.
• Lockout: several failures in a row. Many boards retry on their own after one to three hours.

What can I safely check myself?

Do only the checks that need no tools and no access to the burners:

• Filter: replace it if you cannot see light through it, using the size printed on the frame.
• Registers and returns: open every supply vent and clear the return grilles.
• Power: the furnace switch (it looks like a light switch, on or near the unit) is on, the breaker has not tripped, and the blower door is fully seated.
• Thermostat: HEAT mode, fan on AUTO, fresh batteries if the screen is faint.
• Vent pipes outside: clear snow, ice and leaves from the intake and exhaust pipes of a high-efficiency furnace by hand, after the leaves drop and after every heavy snow.
• One reset: with the code written down, switch the furnace off for about a minute and back on, once. If the same code returns, stop and book a technician.

When do I need a licensed gas technician?

Call a technician when the code returns after these checks and one reset, and straight away for a rollout code, repeated limit codes, or any code with a gas smell, soot or a CO alarm. In Ontario, work on the gas side of a furnace must be done by a Technical Standards and Safety Authority (TSSA) registered fuels contractor using a certified gas technician. That includes the pressure switch, flame sensor, igniter, inducer, gas valve, rollout reset and limit diagnosis. You can check a company on the TSSA registry.

Give the technician the brand and model from the rating plate, the code and its colour, when it happens (first start of the day, every cycle, only in wind or deep cold) and what you already checked.

What does a furnace repair cost on Doyo?

On Doyo, Furnace Repair is priced by written quote, and Doyo checks the technician's TSSA gas certificate before they are offered heating work. If the furnace has not been serviced this year, Furnace Maintenance is $201.25 on Doyo, the full price with Doyo's fee included. A yearly service is when faults such as a dirty flame sensor or a plugged condensate drain tend to get caught before winter.