The first cold morning arrives, you turn up the thermostat, and nothing happens. Before you call for a repair, a few quick checks can save you a service visit. Many “dead furnace” calls in the Valley turn out to be a setting, a switch or a dirty filter.
Smell gas? Don’t touch the thermostat or any switches. Leave the house and call SoCalGas or 911 from outside.
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1. Check the thermostat
Make sure it’s set to Heat, not Cool or Off, and that the temperature is set a few degrees above the room temperature. If the screen is blank or dim, replace the batteries. A thermostat that’s been on cooling all summer is the most common reason a heater “won’t turn on.”
2. Check the breaker and the furnace switch
Look in your electrical panel for a tripped breaker labeled furnace or heater. Most furnaces also have a switch that looks like a light switch on or near the unit, often in a hall closet, attic or garage. It sometimes gets flipped off by accident.
3. Make sure the furnace door is closed
Furnaces have a safety switch that keeps them off if the front panel isn’t seated all the way. If someone changed the filter recently, check that the panel is pushed in and latched.
4. Look at the air filter
A badly clogged filter can make the furnace overheat and shut itself off for safety. If the filter is gray and caked with dust, replace it. Valley dust and smoke from fire season clog filters faster than you might expect.
5. Check the gas valve
The handle on the gas line going into the furnace should run in the same direction as the pipe. If it’s turned across the pipe, the gas is off.
6. Read the status light
Most newer furnaces have a small light you can see through a window in the panel. A blinking pattern is an error code, and the code chart is usually printed inside the furnace door. Write it down: it helps the technician.
7. High-efficiency furnace? Check the drain
High-efficiency furnaces produce water and drain it through a small tube. If that drain clogs, a safety switch can stop the furnace from running.
When to stop and call a pro
- You smell gas, or a carbon monoxide alarm goes off. Get everyone outside and call 911.
- The breaker trips again as soon as you reset it.
- The furnace starts, then shuts off over and over.
- You hear banging, squealing or rumbling, or see scorch marks.
- Everything above checks out and it still won’t run.
Don’t keep resetting a furnace that keeps shutting down. See our guides to furnace repair, a pre-winter tune-up and a heater that smells like burning.
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Call us and a real person will connect you with an independent, licensed heating contractor who serves your part of the San Fernando Valley. It’s free for homeowners.
Near you: Furnace and heating service in Sunland-Tujunga · HVAC services in Porter Ranch · HVAC services in North Hollywood
Did a gas company technician shut your furnace off as unsafe? See what a “red tag” means and what to do next.
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