Furnace Not Heating? 5 Common Causes and How to Fix Them
When your furnace stops producing heat in the middle of a cold snap, the instinct is to call an HVAC technician immediately. But many furnace problems have simple causes that a homeowner can diagnose and fix without a service call. Before you spend $150–$300 on a repair visit, run through these five common issues.
1. Thermostat Issues
It sounds obvious, but start here. Check that your thermostat is set to HEAT mode (not COOL or OFF), the temperature is set above the current room temperature, and the batteries aren’t dead. If you have a programmable thermostat, verify the schedule hasn’t changed. Try switching to a higher temperature and waiting a few minutes. If nothing happens, try switching the fan to ON — if the blower runs but there’s no heat, the issue is with the furnace itself, not the thermostat.
2. Dirty or Clogged Air Filter
A clogged air filter is the single most common cause of furnace problems. When the filter is blocked, airflow drops and the furnace can overheat, triggering the high-limit safety switch to shut it down. Pull out the filter and hold it up to a light — if you can’t see through it, replace it. We stock all standard filter sizes at our Dallas location. As a rule of thumb, check your filter monthly during heating season and replace it every 1–3 months.
3. Faulty Hot Surface Ignitor
If your furnace clicks on and you hear the inducer motor running, but it never actually ignites, the hot surface ignitor (HSI) is likely the problem. The ignitor is a small, fragile silicon carbide or silicon nitride element that glows red-hot to light the gas. Over time, they crack or burn out.
You can visually inspect the ignitor by removing the furnace’s lower access panel and watching through a cycle. If it doesn’t glow orange/red when the furnace tries to light, it needs replacement. This is a common DIY repair — the part typically costs $15–$40 and takes about 20 minutes to swap. We carry ignitors for most furnace brands in our furnace and heating parts section. For a detailed walkthrough, see our guide on diagnosing and replacing a hot surface ignitor.
4. Tripped Flame Sensor
The flame sensor is a safety device that confirms the burner has actually lit. If the sensor is dirty (covered in carbon buildup), it can’t detect the flame and will shut the gas valve off within a few seconds of ignition — meaning the furnace lights briefly then shuts right back down.
Cleaning a flame sensor is one of the easiest HVAC repairs. Remove the single screw holding the sensor, pull it out, and gently scrub the metal rod with fine-grit sandpaper or steel wool until it’s shiny. Reinstall and test. If it still fails, the sensor may need replacement — we stock them for most brands.
5. Blower Motor Problems
If the furnace ignites and produces heat but the warm air isn’t circulating, the blower motor may be the issue. Listen for unusual sounds — grinding, squealing, or rattling. A blower motor that overheats can trip its internal overload protection and shut down. Sometimes the fix is as simple as a seized bearing or a worn belt (on older models). For ECM blower motors, the control module can also fail separately from the motor itself.
When to Call a Professional
If you smell gas, hear a loud bang when the furnace ignites, or the problem involves the gas valve or heat exchanger, call a licensed HVAC tech. These are safety-critical components that require professional training and tools. For everything else — filters, ignitors, flame sensors, capacitors, and blower issues — a confident DIYer can handle the repair with the right parts and guidance.
Get the Parts You Need
Open To Public HVAC Parts carries ignitors, flame sensors, blower motors, filters, thermostats, and everything else you need for furnace repair — all available for walk-in pickup at our Dallas store. Browse our online catalog, watch our free HVAC video tutorials, or check out all the brands we carry.
