A furnace that does nothing at all, no blower, no click, no attempt, is usually the easiest heating problem to fix and the one that makes people panic most. Most of the time the cause is upstream of the furnace: a switch, a breaker, a thermostat, a safety that tripped for a reason. Here are the seven things we check, in order, and about half of them you can check yourself in ten minutes without touching the furnace.
The seven checks
- The thermostat
- The breaker
- The power switch on the furnace
- The blower door
- The condensate float switch
- The gas supply
- The lockout light
1. The thermostat
Set to HEAT, not OFF or COOL. Set temperature above the room temperature. If the screen is blank or dim, the batteries are dead; most thermostats take two AAs behind the faceplate and a dead thermostat is a dead furnace. If it is a smart thermostat with no batteries, it may have lost its wired power because of one of the items below. Also check the schedule: a thermostat set to 60 degrees at night by someone two owners ago will make a furnace look dead at 6 a.m.
2. The breaker
Furnaces are on their own breaker, usually 15 or 20 amps, labeled FURNACE or FAU or sometimes nothing helpful at all. A tripped breaker sits halfway between ON and OFF. Push it fully off, then on. If it trips again immediately, stop and call; something in the furnace is shorting and resetting it repeatedly is how motors and boards get damaged.
3. The power switch on the furnace
Every furnace has a switch on or next to it that looks exactly like a light switch. It is on the furnace cabinet, on the wall beside it, or on the joist above an attic unit. People turn it off by accident constantly, especially in closets and garages where it looks like it controls a light. Flip it. On attic furnaces in Natomas, Elk Grove and Roseville, it is often right at the top of the attic ladder, where someone reaching for the light finds it instead.
4. The blower door
Behind the lower front panel of the furnace there is a small button switch that cuts all power when the panel is removed. If the panel was taken off to change a filter and put back a quarter inch off, the switch is not pressed and the furnace is dead. Take the panel off and put it back on firmly. You will often hear the furnace come to life the moment it seats.
5. The condensate float switch
If your furnace has white PVC drain pipes, it is a condensing furnace and it makes water as it runs. That water drains through a trap, and on attic units through a pan under the furnace with a float switch in it. If the drain clogs, water backs up, the float rises, and the switch shuts the furnace off to prevent a ceiling leak. Look in the pan. If there is standing water, that is your answer, and the fix is clearing the drain, not the furnace. The same switch shuts off the air conditioner in summer; our article on an AC that will not turn on covers the same check from the other season.
6. The gas supply
Ten seconds. Is the handle on the gas pipe to the furnace parallel to the pipe, meaning open? Do the stove burners light? If PG&E has worked on the street or a bill went unpaid, the gas may be off. A furnace with power but no gas will start its sequence, run the inducer fan, try to light, fail, and shut down. That is different from doing nothing at all, and it points to gas rather than power.
7. The lockout light
Most furnaces built since the late 1990s have a small LED visible through a window in the blower door. If it is blinking a pattern, the furnace has locked itself out after failing to light several times, and the pattern is a code. The legend is on a sticker inside the door. A lockout clears when you turn the power switch off for thirty seconds and back on. If the furnace then lights and runs, the underlying fault, usually an igniter, flame sensor or pressure switch, is still there and will lock it out again. If it runs, that is the moment to read our checks for a furnace blowing cold air, because that is what a failing igniter looks like before it becomes this.
What it usually turns out to be
When a homeowner has been through the list above and the furnace is still dead, the causes we find most often are these, roughly in order:
- A failed transformer or a blown low-voltage fuse on the control board. The board has a small automotive-style fuse, usually 3 or 5 amps, and a thermostat wire that shorted, often where it runs through the attic, blows it. Cheap part, but the short has to be found.
- A failed control board. No lights, no response, power confirmed at the board. This is the one real repair on the list.
- A failed thermostat. Less common than people think, but it happens, and it is quick to confirm by jumping the terminals.
- Rodent damage to wiring. Crawlspace furnaces in the older Sacramento neighborhoods and attic units everywhere. Rats chew low-voltage wire.
Two things not to do
- Do not keep resetting a breaker that keeps tripping. Once is a check. Twice is a diagnosis: something is shorting.
- Do not bypass a safety switch to get heat. The door switch, float switch and rollout switches are there for a reason. Taping a door switch closed to run a furnace with the panel off has put carbon monoxide in more than one house.
When to call and what it costs
If you have checked the thermostat, breaker, power switch, blower door and float pan, and the furnace is still doing nothing, the rest is ours. Our diagnostic is a flat $69 across the Sacramento area, credited toward the repair. Same-day service in Roseville, Rocklin, Sacramento, Citrus Heights, Folsom and Elk Grove. In Roseville, see furnace repair in Roseville; in the city, furnace repair in Sacramento. We also have dedicated pages for Rocklin, Folsom, Elk Grove and Citrus Heights. The full page is furnace repair.
If the furnace does start but then stops, over and over, that is a different list: see why a furnace keeps turning on and off. And if it starts with a bang, read our guide to furnace noises before running it again.
Frequently asked questions
Why is my furnace not turning on at all?
Most often a thermostat problem, a tripped breaker, the furnace power switch flipped off, or the blower door not seated so its safety switch is open. On condensing furnaces, a float switch in the drain pan shuts the unit off when the drain clogs. Check those five before calling.
Where is the furnace power switch?
On the furnace cabinet, on the wall next to it, or on a joist above an attic unit. It looks exactly like a light switch, which is why it gets turned off by mistake so often.
My furnace light is blinking. What does it mean?
The furnace has locked itself out after failing to light, and the blink pattern is a code listed on a sticker inside the blower door. Turning the power off for thirty seconds clears the lockout, but the fault that caused it, usually an igniter or flame sensor, is still there.
Can I reset my furnace myself?
Yes. Turn the power switch on the furnace off, wait thirty seconds, and turn it back on. If it starts and runs, the lockout is cleared for now. If it locks out again within a day, call; the part that caused it needs attention.
How much does it cost to fix a furnace that will not turn on?
Our diagnostic is $69 and comes off the repair. A blown low-voltage fuse, a thermostat or a cleared drain are short visits. A control board or transformer costs more. If the furnace is past fifteen years and the board has failed, we price a replacement alongside so you can compare.