Furnace Not Turning On: What to Check and When to Call a Pro

A furnace that will not turn on is stressful, especially on a cold night. Many no-heat calls turn out to be something simple, like a thermostat setting or a switch that was turned off by mistake. This guide covers the safe checks you can do in a few minutes and the signs that mean you need a professional.

The checklist below comes from FortisBC, Technical Safety BC and furnace makers Trane and Carrier, checked on September 25, 2026. Your owner's manual always comes first.

Step-by-step checks you can do safely

1. Check the thermostat mode. Make sure it is set to "Heat", not "Off", "Cool" or "Fan". Set the temperature a few degrees above the current room temperature. Trane notes that some thermostats need you to press a button or move a switch before heat comes on.
2. Replace the thermostat batteries. FortisBC calls dead batteries in a digital thermostat "an easy fix" that is often missed. If the screen is blank or shows an error code, check the thermostat manual.
3. Check the furnace power switch. Even a gas furnace needs electricity to run its fan and controls. Trane says to look for a switch like a light switch on the side of the furnace, on a nearby wall, or on a ceiling beam. Make sure it is "On".
4. Check the breaker. Look in your electrical panel for a tripped breaker for the furnace. Reset it once. Trane says that if it trips again, you have an electrical problem and should call for service.
5. Check the air filter. A clogged filter blocks airflow. FortisBC says to check your filter at least every three months and replace or clean it if it is dirty. Turn off the furnace switch and breaker before you change it.
6. Check the furnace doors. FortisBC lists "Are the furnace doors and access panels all closed?" as a key check. Trane notes that some furnaces have a blower door switch that cuts power and gas when the door is removed, so a loose panel can stop the furnace from running.
7. Check vents and registers. Make sure supply vents and return air grills are open and not blocked by furniture, rugs or curtains.

High-efficiency furnaces: look for water

High-efficiency (condensing) furnaces make water as they work. The water drains away through a small plastic condensate line. Trane says you can usually tell you have this type because the exhaust pipe is white PVC plastic, often paired with a second PVC intake pipe. A standard furnace has a metal exhaust pipe.

If you see water pooling around the furnace, Carrier and Trane say a clogged condensate drain line is one common cause. Other causes include a condensate pump problem, a humidifier leak, or a cracked heat exchanger. Turn the system off at the thermostat, mop up the water, and call an HVAC or gas professional. Water near electrical parts is a safety risk.

Blinking lights and error codes

Many newer furnaces have a control board with a small light that blinks in a pattern, or shows a code. Trane explains that the meaning depends on the make and model. A slow green blink may just mean there is no call for heat. Red blinking lights generally signal a problem.

• On some furnaces you can see the light through a small window. Trane says not to take the front panel off while the furnace is running.
• Write down the colour and blink pattern (for example, "red, 3 blinks, pause").
• Look up the code in your owner's manual.
• Give the code and the furnace brand and model to the technician when you book.

Trane warns homeowners not to try to fix the furnace themselves because of electrical hazards, hazardous gases and the risk of explosion. It can also void your warranty.

Pilot light and ignition

FortisBC notes that newer gas furnaces may not have a standing pilot light. Trane says newer furnaces use electronic ignition, while older ones may have a pilot light. A working pilot should show a small blue flame. Trane says a yellow flame means you should call for repair.

If an older furnace's pilot is out, follow only the lighting instructions on the furnace label. Trane says a pilot that will not stay lit can be caused by a draft, a dirty pilot or a faulty part such as a thermocouple. Call a professional to fix it. On newer furnaces with electronic ignition, a technician needs to find out why the igniter is not working.

Some furnaces go into "lockout" after several failed attempts to light. Trane says a hard lockout needs a technician to find and fix the cause, then reset the furnace. Check your manual before trying any reset.

When to call a licensed gas contractor

Call a professional if the checks above do not work, or if you notice any of these warning signs listed by FortisBC:

• Any gas smell (call FortisBC's emergency line first) or a sharp smell that makes your eyes sting.
• Headaches, nausea, tiredness or other flu-like symptoms at home.
• Frequent pilot light outages or delayed ignition.
• A yellow or wavering flame.
• Soot, streaks or scorch marks on the furnace or vent.
• Too much or too little heat.

In BC, gas work must be done by qualified people working for licensed gas contractors, unless a homeowner qualifies for a homeowner permit. Technical Safety BC and FortisBC both recommend yearly servicing. FortisBC gas customers can get a $50 rebate for a furnace or boiler service by a licensed gas contractor. Apply within 60 days of the invoice.

Carbon monoxide safety

A furnace that is not burning properly can produce carbon monoxide (CO). You cannot see or smell it. FortisBC lists symptoms such as headaches, nausea, drowsiness, impaired judgment and loss of coordination. If you feel better when you leave the house, that can be a sign of CO.

If your CO alarm sounds or you suspect CO, FortisBC says to turn off your appliances, get everyone including pets outside, call 911 or 1-800-663-9911, and get medical help for any symptoms.

• Install CO alarms. The Province of BC says every home with a fuel-burning appliance, attached garage or fireplace should have one, ideally on every level and outside each sleeping area.
• Buy certified alarms. FortisBC says to look for CSA approval and the standard number 6.19 on the label.
• Test and replace. Technical Safety BC says to test CO alarms monthly and replace the unit every 7 to 10 years.

Technical Safety BC notes that CO alarms are not yet required by law in every BC community. It has consulted the public on possibly requiring CO alarms wherever gas appliances are used in homes.

Frequently Asked Questions

Official sources

• 5 furnace flops and when to fix them — FortisBC
• Appliance maintenance and safety — FortisBC
• Gas leaks and odours — FortisBC
• Carbon monoxide safety — FortisBC
• Appliance maintenance rebates — FortisBC
• Carbon Monoxide Safety — Technical Safety BC
• Gas Code Adoption and CO Alarm Requirements consultation — Technical Safety BC
• Annual Servicing for Gas Appliances — Technical Safety BC
• Home Modifications — Technical Safety BC
• Carbon monoxide safety — Province of British Columbia
• Furnace Not Turning On — Trane
• Furnace Light Blinking — Trane
• Furnace Leaking Water — Trane
• Furnace Leaking Water — Carrier Canada

General information only, not professional advice. Rules and prices change, so check the official sources above before you act. Spotted something out of date? Tell us.