This one needs a licensed pro
Likely cause: an ignition, flame sensor or control fault
With settings, filter, power and fuel ruled out, the fault is inside the furnace. Note any blinking light and call an HVAC technician.
Why this happens
Carrier lists a pilot that's out, the limit switch, the flame sensor and control failures after the simpler causes, and Lennox and Trane list the flame sensor, igniter, control board and a clogged condensate line. These parts sit behind the furnace's panels next to gas and high-voltage wiring. Trane notes that DIY repairs on a furnace under warranty can void the warranty.
What to do right now
- Look for a blinking light through the small window in the furnace door. Count the blinks and note the color; Carrier says these are diagnostic codes. Don't remove the door to look.
- Set the thermostat to Off so the furnace stops blowing cold air.
- Don't try to clean, relight or adjust anything inside the furnace.
- Write down the brand, model and serial from the furnace's data plate.
- If you smell gas at any point, leave the house and call 911 or your gas utility from outside.
What to tell the HVAC technician
- The blinking light pattern: color, number of blinks, and any pause.
- That the thermostat, filter, power switch, door panel and fuel all checked out.
- Whether the furnace starts and then shuts off, or never lights at all.
- The brand, model, serial and approximate age of the furnace.
Build the full list: what to tell the HVAC technician.
Start this troubleshooter over
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