Furnace won't run

Hoping I can get some troubleshooting before I have to call for help. My thermostat indicates the heat should be on, but it is not. No indicator lights inside the furnace that I can see. It does not have a pilot. When I flipped some switches and the breaker it ran for a few seconds then shut off. Not sure what else could be the deal.

N.B. I replaced the batteries in the thermostat.

Gas ? Oil? How old? Etc.

If less than 10 years old may have trouble code lights inside.



Looks like 2009 (I just moved in, and rent).

I don’t see any indicator lights.

Something dirty or bad sensor is possible. You say it runs for a few seconds, is that just the fan or is there a flame?

On a call for heat, the induction blower should start (ID BLWR on your schematic) and the hot surface igniter should light up. If those events occur, after about 20-30 seconds the furnace should light off. Is any of that happening?

Also, make sure the furnace door is on properly. There is a door switch that must be closed to allow operation.

You are renting, can’t you call the landlord?

After looking again at the schematic, I see the failure codes on the left lower part of the drawing. Find the light (usually an LED) and see if it gives you a flashing signal. Count the flashes and see what the problem is.

Indicator lights are usually found by removing the lower panel.
Take off the top panel, then you’ll find out how to remove the bottom one. It might just pop off, you might have to lift it slightly, you might have to remove one or two screws.

In that section (where the blower is), you’ll see a metal box. You might see a little window. If there is a window, look for a flashing light. If not, pop the cover off. Be careful, it’s all live in there. Check for a flashing light and find the legend. It’ll be mounted somewhere. Either on the panel for that box. On one of the panels you just removed or the inside wall of the furnace somewhere. Otherwise it’ll be in the internet somewhere.
That light will go a long way in helping us.

Random guess is flame sensor, but I’d have more questions. The biggest one is, did it ever light?
Turn it off (at the furnace), wait a minute or so, turn it back it. It should try to run.
A small blower will run, then the gas will start up, then it should light, then the whole thing should die. If you saw fire for a few seconds, it’s the flame sensor. That’s an easy fix.

If the small inducer motor ran but you never got fire, it’s something else. Could be the pressure switch. But we’ll worry about that later.

In the mean time, if this is the type of furnace that vents to the side of your house via PVC pipes, go make sure neither of them are blocked by snow. Also, as long as your out there, make sure there isn’t snow packed around your gas meter. There’s a diaphragm that has a part pointed down, if that gets snow packed under it, it’ll cause problems.

If you’re renting (as I see someone else said), call the landlord, call now, the later it gets, the harder it is to fix as stores start closing.

Also, you said it ran for a few seconds. I want to clarify that it was, in fact, seconds. Seconds is different than minutes in terms of which sensor is causing the problem.

Joey P, the OP has a link to the schematic and the fail codes in post 4.

I had both panels off, couldn’t see any lose wires, indicator light wasn’t flashing but there was a piece of electrical tape between the two panels pushing the safety switch. This jerry-rig must have finally failed. Should have used duct tape.

Well, make sure the door switch is making, and see if the furnace will light off properly.

The door switch is 120 volts so be careful there.

I looked through that entire manual, the fault codes are in a different book.

No light, no heat…are you sure the breaker (and the switch on the furnace) is on?

ETA, service manual says the LED will stop of the door button is open.
Hold the door switch in (hence the tape) and see if it starts blinking.

HVAC stores even sell a clip just for that purpose.

When you click on the link, go to page 15 and the codes are in the lower left corner of the schematic.

I’m assuming the OP has the info for the right furnace.

OP said earlier that after switching breaker on/off the furnace ran for a few seconds. Guessing it was the inducer fan starting up. After that, who knows?

My furnace also came on for a very short time, then quit.
I found that the connection to the thermal sensor was bad. Cleaned it. Cleaned the sensor probe. Then it worked fine.
The furnace will shut down if it does not detect that the gas is burning and producing heat. It assumes the gas failed to light. This happens in older furnaces, when the sensor for the pilot light fails. The gas valve shuts off. But those ones don’t even try to start at that point.

When we had these symptoms, the problem was that the flu baffle wasn’t opening. Modern furnaces open and shut a baffle in an exhaust leading out to the flu when the furnace ignites and shuts off. If the baffle doesn’t open, the furnace shuts off.

I went through this a few years ago.

For gas furnace:

  1. Start small (inducer) motor, check that flue is open.
  2. Open gas valve
  3. Heat ignitor
  4. Send current to flame detector.
  5. Was flame found?

If any of those fail, no heat for you.

Grsz11, did you get the heat on?