Central Texas winters are mild compared to the rest of the country, but when temperatures drop into the 30s and 40s — which happens more than people expect in places like Lexington, Giddings, and Taylor — you need your furnace to work. If it's blowing but the air isn't warm, here's how to diagnose the problem.
Check These First (No Tools Required)
Thermostat settings. This catches people more often than you'd think. Make sure your thermostat is set to "heat" (not "cool" or "off") and the temperature is set above the current room temperature. Also check the fan setting — if it's set to "on" instead of "auto," the fan runs continuously, including between heating cycles. During those off cycles, the air coming from the vents feels cool because it's just circulating room-temperature air.
The fix: Switch the fan from "on" to "auto." In auto mode, the fan only runs when the furnace is actively producing heat.
Air filter. A severely clogged filter restricts airflow to the point where the furnace overheats internally. Modern furnaces have a safety limit switch that shuts off the burners when this happens — but the fan keeps blowing. So you get airflow but no heat. This is the furnace protecting itself from cracking the heat exchanger, which would be a far more expensive problem.
The fix: Pull out the filter and check it. If it's gray, matted, or you can't see light through it, replace it. If the furnace has shut off on the limit switch, replacing the filter and letting the system cool down for 30 minutes will usually allow it to restart normally.
Common Causes That Need Diagnosis
Pilot light is out (older furnaces). If your furnace is an older model with a standing pilot light — common in homes built before 2000 — the pilot light may have gone out. Without the pilot flame, the burners can't ignite. The fan may still run (depending on the system), but no heat is produced.
The fix: You can relight a pilot light yourself by following the instructions on the furnace's access panel. Turn the gas valve to "pilot," hold down the reset button, and use a long lighter to ignite the pilot. Hold the button for 30 seconds to let the thermocouple heat up, then release. If the pilot won't stay lit, the thermocouple is likely bad — that's a $150–$250 professional repair.
Ignitor failure (newer furnaces). Most furnaces installed in the last 20 years use a hot surface ignitor instead of a pilot light. This is a small ceramic element that glows red-hot to ignite the gas. Ignitors are fragile and wear out over time — they typically last 3–7 years. When the ignitor fails, the furnace goes through its startup cycle but the burners never light.
The fix: Ignitor replacement is a common furnace repair. The part itself costs $30–$80, and professional replacement (including labor) typically runs $150–$300. This is not a good DIY repair because it involves working near the gas valve and burner assembly.
Dirty flame sensor. The flame sensor is a small metal rod that sits in the burner flame. Its job is to confirm that gas is actually burning — if it doesn't detect a flame within a few seconds of the gas valve opening, it shuts the gas off as a safety measure. Over time, the sensor gets coated with carbon buildup and can no longer detect the flame, even though the burners are lit (briefly). The furnace lights, runs for 3–10 seconds, then shuts off. The fan keeps blowing cold air.
The fix: A technician removes the flame sensor and cleans it with fine sandpaper or steel wool. This is one of the most common heating repairs we perform — it's quick, inexpensive, and immediately solves the problem. Most techs charge $100–$200 for this service.
Gas supply issue. If the gas supply to the furnace is interrupted — a closed valve, a problem with the gas meter, or a gas line issue — the furnace can't produce heat. Check that the gas valve near the furnace is in the "on" position (lever parallel to the pipe). If your gas stove or water heater also isn't working, the issue is upstream of the furnace.
The fix: If the gas valve at the furnace is open and other gas appliances aren't working either, contact your gas utility company. If only the furnace is affected, the issue may be the furnace's internal gas valve — a repair that requires a licensed technician.
Cracked heat exchanger. This is the most serious diagnosis on this list. The heat exchanger is the metal chamber where combustion gases heat the air that flows into your home. If it cracks, the furnace's safety systems shut it down to prevent carbon monoxide from entering your ductwork. Some furnaces will flash error codes on the control board when this happens.
The fix: A cracked heat exchanger generally means the furnace needs to be replaced. Repair is technically possible on some models, but the cost (often $1,500–$3,000) usually exceeds the value of the furnace, especially on older units. If your furnace is under 15 years old, the heat exchanger may still be under warranty — check your documentation.
Furnace Error Codes
Most modern furnaces have a small LED light on the control board (visible through a window on the front panel) that flashes a diagnostic code. Check your furnace's manual or the chart typically printed inside the access panel. Common codes indicate ignitor failure, flame sensor fault, limit switch trip (overheating), or pressure switch failure. Reporting the error code to your technician speeds up the diagnosis.
When to Call for Heating Repair
If you've checked the thermostat and filter and the furnace still isn't producing heat, it's time for a professional. Don't run the system repeatedly trying to get it to work — each failed ignition cycle can flood the combustion chamber with unburned gas, which is a safety concern.
At Kimco Plumbing & Air, we provide heating repair across Central Texas. Flat-rate pricing, no overtime charges, and same-week scheduling. Call (737) 260-7255.