Furnace Not Heating? 7 Things to Check First

Published June 30, 2026 - Cavinder Plumbing, Heating and Cooling - Granger, IN

Quick answer: Before you call for furnace repair, check these seven things in order: (1) the thermostat - mode set to heat, batteries, and setpoint above room temperature; (2) the breaker for the furnace; (3) the air filter for clogging; (4) the furnace power switch (it looks like a light switch near the unit); (5) the gas supply and pilot or igniter; (6) blocked vents and registers; and (7) the condensate safety switch on high-efficiency furnaces. A clogged filter, a flipped power switch, or a tripped condensate switch are the most common DIY-fixable causes. If those all check out, the issue is likely the flame sensor or another internal part, and it's time to call. Here's how to work through each one safely.

No heat and can't get it back on? Call (574) 633-4557. Cavinder offers 24/7 emergency furnace service in Granger, South Bend, Mishawaka, Elkhart, and across Michiana.

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1. Check the Thermostat

It sounds obvious, but the thermostat is the single most common reason a furnace "won't heat," and it's the easiest to rule out.

• Mode: Make sure it's set to "Heat," not "Cool" or "Off." It's easy to leave it on the wrong mode after the shoulder season.
• Setpoint: Raise the target temperature a few degrees above the current room temperature so the furnace gets a clear call for heat.
• Batteries: Many thermostats run on batteries. A dead or dying battery can blank the screen or stop it from signaling the furnace. Replace them.
• Schedule and hold: If you have a programmable or smart thermostat, confirm a schedule or "hold" hasn't set the temperature low.

If the screen is blank even with fresh batteries, that points to a wiring or power issue worth a technician's look.

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2. Check the Breaker

Furnaces need electricity to run the blower and controls, even gas furnaces. A tripped breaker cuts that power entirely.

Go to your electrical panel and look for a breaker labeled "furnace," "heat," or "air handler." If it's tripped (sitting between on and off, or fully off), switch it fully off and then back on. If it trips again right away, stop - a breaker that won't stay set usually means an electrical fault that needs to be diagnosed, not reset repeatedly.

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3. Check the Air Filter

A clogged filter is one of the most common no-heat and weak-heat causes we see. When the filter is choked with dust, airflow drops, the furnace overheats, and a safety limit shuts it down to protect the heat exchanger. The furnace may run briefly, then quit, or short-cycle on and off.

Pull the filter and hold it up to the light. If you can't see light through it, replace it. In Michiana, a 1-inch filter should be checked monthly and changed every one to three months. A fresh filter alone resolves a surprising number of no-heat calls.

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4. Check the Furnace Power Switch

There's a switch that looks exactly like an ordinary light switch on or near the furnace, often at the top of the basement stairs or on the unit's housing. It cuts power to the furnace. It gets flipped off by accident more often than you'd think - by someone doing laundry, carrying boxes past it, or assuming it's a light switch.

Make sure it's on. Give the furnace a minute to go through its startup sequence after you flip it.

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5. Check the Gas Supply, Pilot, or Igniter

If the blower runs but the air is cold, the furnace may not be lighting. A few things to check safely:

• Gas supply: Confirm the gas shutoff valve on the line to the furnace is open (handle parallel to the pipe), and that other gas appliances (stove, water heater) are working. If nothing gas-powered is working, contact NIPSCO.
• Older furnaces with a standing pilot: If the pilot is out, follow the lighting instructions printed on the furnace label. If it won't stay lit, the thermocouple may be failing.
• Newer furnaces with electronic ignition: These have a hot-surface igniter or spark igniter instead of a standing pilot. You can't relight these by hand; a failed igniter needs a technician.

Safety first: If you ever smell gas (a rotten-egg odor), do not flip switches or look for the source. Leave the home and call NIPSCO and us from outside.

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6. Check for Blocked Vents and Registers

Furnaces are designed to push air through the whole duct system. When too many supply registers are closed or blocked by furniture, rugs, or boxes, airflow backs up, the system overheats, and a safety limit can shut it down - the same way a clogged filter does.

• Walk the house and open all supply registers. Closing vents to "save energy" in unused rooms often backfires by restricting airflow.
• Move furniture, rugs, and boxes off of registers.
• Check that the large return-air grille isn't blocked either - the furnace needs a clear path to pull air back.

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7. Check the Condensate Safety Switch (High-Efficiency Furnaces)

High-efficiency furnaces (90 percent AFUE and up, the kind with a PVC exhaust pipe) produce water as they run, and that condensate drains away through a small line and often a pump or trap. If that drain clogs or the pump fails, water backs up and a float safety switch shuts the furnace off to prevent overflow. This is common in Michiana basements, and it's a frequent fall no-heat cause.

Look near the furnace for the condensate drain line and any collection box or small pump. Signs the safety switch has tripped include water pooling around the furnace base or in the drain pan. Clearing a clogged condensate line is sometimes a DIY job, but if you're not sure, leave it for a technician - water plus electronics is not a good combination.

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When to Stop and Call

If you've worked through all seven and the furnace still won't heat, the cause is likely internal and needs a technician. The most common next culprit is a dirty or failed flame sensor - a small metal rod that confirms the burners lit. When it gets coated in residue, the furnace lights for a few seconds, then shuts down because it can't "see" the flame, which shows up as repeated short-cycling. Cleaning or replacing the flame sensor is routine for a technician but not a homeowner job.

Stop and call right away if you smell gas, the breaker trips repeatedly, you see scorching or soot, or there's standing water you can't safely clear. Call (574) 633-4557 - we run 24/7 emergency service, so a no-heat night in January doesn't have to wait. See our furnace repair page for what a service visit covers.

While you're here: a furnace that runs but smells off on the first cold day is usually normal. Our post on that first-of-the-season furnace smell explains when it's nothing and when it's worth a look.

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Frequently Asked Questions

Why is my furnace blowing cold air?

The blower is running but the burners aren't producing heat, or aren't staying lit. Common causes are a thermostat set to "on" instead of "auto" (so the fan runs constantly between heat cycles), a furnace that's overheating and shutting off the burners on a clogged filter, a failed igniter or pilot, or a dirty flame sensor causing the furnace to light then quit. Work through the seven checks above, starting with the thermostat fan setting.

Does my furnace have a reset button?

Some older furnaces and blower motors have a reset button, but most modern furnaces reset themselves after you cycle the power switch or breaker off and on. If a furnace keeps tripping and needing a reset, that's a symptom of an underlying problem (overheating, a failing component, a tripped safety) that should be diagnosed rather than reset over and over.

Why is my furnace short-cycling?

Short-cycling, turning on and off every minute or two without warming the house, is usually an overheating or flame-sensing problem. The top causes are a clogged filter restricting airflow, closed or blocked vents, an oversized furnace, or a dirty flame sensor that loses sight of the flame and shuts down. Start with the filter and vents; if it continues, the flame sensor or a control issue likely needs a technician.

Is it safe to keep trying to start my furnace?

Cycling the power once or twice to retry is fine. What's not safe is repeatedly resetting a breaker that keeps tripping, relighting a pilot again and again, or running the furnace if you smell gas or see soot or scorching. Repeated attempts on a furnace with a real fault can make things worse. If it won't stay running after a couple of tries, stop and call.

What is emergency heat and should I use it?

"Emergency heat" (or "aux heat") is a setting on heat-pump thermostats, not standard gas furnaces. It switches the system to its backup heat source - usually electric strips or the gas furnace in a dual-fuel setup - when the heat pump can't keep up or has failed. It's meant as a temporary measure to stay warm while you wait for service, not an everyday setting, because the backup heat is less efficient. If you have a gas furnace with no heat pump, you won't have this setting.

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Still No Heat? We're Available 24/7

Call (574) 633-4557 or book online. Licensed in Indiana (CO19900013 HVAC / PC19700254 plumbing). 24/7 emergency furnace repair in Granger, South Bend, Mishawaka, Elkhart, and across St. Joseph and Elkhart County. We install and service Sure Comfort, Mitsubishi Electric, and Fujitsu Halcyon heating systems.