Furnace Not Heating? 7 Reasons Why — and What to Do in New Lenox, IL

It's 10pm on a January night in New Lenox. You hear the furnace kick on — but the air coming out of the vents is cold. This is one of the most common calls we get every winter, and the good news is that most causes are fixable quickly.

Here are the 7 most common reasons a furnace runs but doesn't heat, in order from most to least common based on what we see in Will County homes.

1. Dirty or Clogged Air Filter

This is the #1 cause of furnace problems in New Lenox — and it's completely preventable. When the air filter gets too clogged, airflow is restricted so severely that the furnace overheats and the high-limit switch shuts it down as a safety measure. The blower keeps running, but the burners are off.

Fix it yourself: Check your filter. If it's gray and clogged, replace it with a new one (1-inch filters should be replaced every 1–3 months). After replacing, turn the furnace off at the thermostat for 30 minutes, then back on.

2. Thermostat Set to "Fan On" Instead of "Auto"

If your thermostat fan setting is on "ON" instead of "AUTO," the blower runs continuously — even when the furnace isn't actively heating. This means you'll feel room-temperature air from the vents when the burners aren't firing.

Fix it yourself: Switch the fan setting from "ON" to "AUTO" on your thermostat.

3. Ignitor Has Failed

Modern furnaces use a hot surface ignitor — a small ceramic element that glows red-hot to light the gas burners. When it fails, the furnace goes through its startup sequence, the gas valve opens, but nothing ignites. The furnace shuts down after 3 attempts and locks out.

Signs: You may hear the furnace "click" and start, then shut off after 30–60 seconds repeatedly. You might also see an error code on the furnace control board.

What to do: This requires a technician. Ignitor replacement in New Lenox typically costs $200–$350 and takes about an hour.

4. Gas Supply Issue

If the gas supply to the furnace is interrupted — a closed shutoff valve, a tripped gas meter, or a supply disruption — the furnace will attempt to light but fail. This is more common after a long period of non-use at the start of the heating season.

Check: Make sure the manual gas shutoff valve near the furnace is in the open position (handle parallel to the pipe). If you smell gas, leave the house immediately and call Nicor Gas at 888-642-6748.

5. Flame Sensor Is Dirty

The flame sensor is a small metal rod that detects whether the burners are lit. Over time, it develops a coating of oxidation that prevents it from reading the flame correctly. The furnace lights, detects no flame (incorrectly), and shuts off the gas as a safety measure.

Signs: Furnace lights briefly (1–3 seconds) then shuts off. This repeats 3 times before lockout.

What to do: A technician can clean the flame sensor in about 30 minutes. This is a common annual maintenance item.

6. Cracked Heat Exchanger

This is the most serious issue on this list. The heat exchanger is the metal component that separates combustion gases from the air circulating through your home. A crack allows carbon monoxide to enter your living space.

Signs: The furnace shuts off shortly after starting, you notice a strange smell, or your CO detector goes off. Some furnaces have a safety switch that detects a cracked heat exchanger and shuts down automatically.

What to do: Do not operate the furnace. Call a technician immediately. Heat exchanger replacement is expensive ($1,000–$2,000+) and on older furnaces, replacement of the entire unit is often more cost-effective.

7. Control Board or Wiring Issue

The furnace control board is the brain of the system. A failed board, a blown fuse on the board, or a loose wire connection can prevent the furnace from operating correctly even when all other components are functional.

Check: Look for a small LED on the control board that flashes an error code. Your furnace manual (or a label inside the furnace door) will tell you what each flash pattern means.

What to do: Control board replacement requires a technician and typically costs $300–$600.

When to Call a Professional

If replacing the filter and checking the thermostat doesn't solve the problem, it's time to call a licensed HVAC technician. New Lenox Heating offers same-day service throughout Will County. We carry common parts on every service van so most repairs are completed in a single visit.