Furnace Won't Turn On? What to Check Before You Call

What does it mean if your furnace makes no sound at all?

What happens when the thermostat calls for heat narrows down the cause fast.

What should I check first when my furnace won't turn on?

Start with the thermostat, then the power to the furnace, the furnace door and the fuel supply. These checks need no tools.

1. Thermostat setting. Set it to HEAT, raise the temperature at least 3 degrees above the room temperature, and wait a minute or two. Many thermostats have a built-in delay before they start the furnace.
2. Thermostat batteries. A blank or dim screen usually means dead batteries. Replace them even if the screen still shows something faint.
3. Breaker and service switch. Check the furnace breaker in your electrical panel, and the light-switch-style service switch on or near the furnace. Either can get switched off by accident, especially when it sits next to a garage light switch.
4. Furnace door. Most furnaces have a safety switch that cuts power when the lower door isn't fully closed. Push the door firmly into place, especially if someone just changed the filter.
5. Gas or propane. Make sure the gas valve at the furnace is open (handle in line with the pipe). In homes on propane, check the tank gauge.
6. Light code. Look through the small window on the furnace door. A blinking light means the control board has logged a fault; count the blinks and write them down.

If the furnace still won't start after these checks, try one reset by turning the service switch off for 30 seconds and back on. If it doesn't start, leave it off and call.

Why won't my furnace turn on?

A furnace that won't start is usually missing power, the start signal, or one of the conditions its safety controls check before lighting. Here are the causes, from most to least likely.

1. Thermostat problem

Dead batteries, the wrong mode, a setpoint below room temperature, or a loose wire behind the thermostat means the furnace never gets the call for heat. A new smart thermostat that was installed without a common wire can also lose power and go blank.

2. No power to the furnace

A tripped breaker, a service switch turned off, or a blown fuse on the control board stops everything. A fuse that blows again after replacement points to a short in the wiring or a failed part, which needs a technician.

3. Furnace door safety switch

If the blower door isn't seated properly, a small push switch keeps the furnace from running. It is a common reason a furnace stops working right after a filter change.

4. Failed igniter

Hot surface igniters crack with age and heat. You may hear the inducer motor start, and then the furnace gives up after a few tries because the burners never light. This is one of the most common no-heat repairs.

5. No gas or an empty propane tank

A closed gas valve, a gas service interruption, or a propane tank that has run low keeps the burners from lighting. Propane is common across rural Fallbrook, Rainbow and De Luz, so check the gauge before you call.

6. Pressure switch, vent or condensate problem

Before lighting, the furnace confirms the vent is clear. A blocked vent, a cracked pressure-switch hose, a weak inducer motor or a full condensate trap on a high-efficiency furnace stops the start-up. After a windy stretch, check the outside PVC pipes for leaves and debris.

7. Safety lockout

After several failed starts, many furnaces lock themselves out for an hour or until power is reset. A lockout is the furnace protecting itself; the real cause is still one of the problems above. If it keeps shutting off instead of not starting, see why a furnace keeps shutting off.

8. Control board failure

A failed control board can leave the furnace completely dead even with power and a working thermostat. Power surges and age are the usual causes. A technician can test it before it is replaced.

When should I stop and call for furnace repair?

Call if you smell gas or burning, if a fuse or breaker keeps blowing, or if the furnace still won't start after the checks and one reset.

• You smell gas or rotten eggs. Leave the house and call 911 from outside, then your gas company or propane supplier.
• The breaker or the control board fuse trips again after you reset it.
• You smell burning or electrical odors, or see scorch marks.
• The furnace clicks and tries to start but never lights.
• There is a blinking light code you can't clear.

Do I need a new furnace if it won't turn on?

Usually not. A thermostat, igniter, switch, fuse or control board is a repair, even on an older furnace. Replacement starts to make sense when an older furnace needs a major part such as a heat exchanger or a blower motor and a control board at the same time. The furnace replacement page explains how to compare the two. A yearly furnace cleaning helps catch worn igniters and sensors before the first cold night.

Furnace won't turn on: FAQs