Furnace Not Working? A No-Heat Checklist for York County Homes

The first cold snap of late October is when York County furnaces that sat all summer get their first real test, and many "no heat" calls turn out to be something a homeowner can check in ten minutes. Work down this list in order; each step rules out the cheapest, most common cause before the next one.

1. Thermostat

• Mode is Heat (not Cool or Off) and the setpoint is a few degrees above the room temperature. On a programmable or smart thermostat, check that a schedule or "away" mode has not pulled the setpoint down.
• If the screen is blank or dim, replace the batteries, even if it also has wired power. Give the furnace up to five minutes after any change; many systems hold a short delay before the burner lights.
• Heat pump owners: "Emergency heat" or "Aux" showing constantly in mild weather is a sign something is wrong, not a normal mode. Set it back to Heat and note it for the technician.

2. Power: the breaker and the furnace switch

• Find the furnace's breaker in the electrical panel. A tripped breaker often sits in the middle, not fully off. Turn it fully off, then on again, once. If it trips again, stop and call a licensed contractor; a breaker that trips twice is reporting an electrical fault, not a nuisance.
• Look for a plain light switch on or next to the furnace, often with a red or metal cover plate. It is the furnace's service switch, and it gets flipped by mistake more often than you would think. Make sure it is on.
• Gas furnaces also need power for the blower, the igniter and the control board, so "no power" is a no-heat cause even when the gas is fine.

3. Filter

Pull the filter and hold it up to a light. If you cannot see light through it, replace it. A clogged filter starves the furnace of air, the heat exchanger overheats, and a safety limit switch shuts the burner off, sometimes after a few minutes of heat, so the pattern is "it runs, then stops, then tries again." The furnace will not settle down until the airflow does. Note the arrow on the new filter's frame; it points in the direction of airflow, toward the furnace.

4. Vents and registers

• Open the supply registers in the rooms that are cold and make sure furniture, rugs or curtains are not covering them or the return grilles. Closing many registers to "push heat" elsewhere raises the pressure in the ducts and can trip the same limit switch as a dirty filter.
• Outside, check the furnace's intake and exhaust pipes (white PVC on a high-efficiency furnace, a metal flue on an older one). Leaves, a wasp nest or a drift of dryer lint over the intake will shut a modern furnace down on a pressure-switch fault.

5. Gas supply, pilot and igniter

• The gas valve on the pipe into the furnace is on when its handle runs along the pipe, off when it sits across it. Check it, and check that other gas appliances (water heater, range) are working, which tells you the house has gas service.
• Older furnaces have a standing pilot light. If it is out, follow the lighting instructions printed on the furnace's own label exactly, and only once. If it goes out again, leave it off and call a professional: a pilot that will not stay lit usually means a failing thermocouple, which is a service job.
• Newer furnaces use a hot-surface igniter or a spark. Watch (do not touch) through the sight glass: a glow or a click with no flame, or a flame that lights and dies within seconds, points to the igniter or the flame sensor. Those are quick, common repairs for a licensed contractor; they are not homeowner parts. Never bypass or jumper a safety control to get a burner to light.

6. What the blinking light means

Many furnaces built in the last twenty years have a small LED on the control board, visible through the sight glass, that blinks a fault code when the furnace locks out. The legend is printed inside the front panel. Codes differ by brand, so this guide does not list them, but the categories are consistent: a steady light is normal; a count of blinks points to a pressure switch, a limit (overheat), a flame-sense failure, an igniter failure or a polarity or grounding problem. Write down the pattern before you cut the power, because resetting the furnace clears it, and tell the technician. A furnace that is locked out after several ignition attempts will usually retry on its own after an hour; turning the power off for thirty seconds and back on resets it sooner. If it locks out again, stop resetting it.

7. When to call a professional

• Any gas smell or CO alarm (see the top of this page), scorch marks, a burning-plastic smell, or water around the furnace.
• A breaker that trips twice, a pilot that will not stay lit, an igniter that glows without lighting, a flame that lights and dies, or a lockout that repeats after one reset.
• Anything behind the furnace's panels: the gas valve, the heat exchanger, the inducer motor, the blower motor and the control board. In South Carolina, gas and refrigerant work is licensed-contractor work; SC LLR issues the HVAC license and has an online lookup. Ask the company for the fault code they found and what they replaced.

Repair pages with every listed company and tap-to-call: York, Rock Hill, Fort Mill, Clover, Tega Cay, Indian Land. Related reading: furnace repair in York County, heat pumps and the seasonal maintenance checklist. We list companies, not services, and we have not checked which jobs each one takes. Call to confirm a company works on your type of system before you book.