A modern furnace runs a strict startup sequence, and knowing where the sequence breaks is most of the diagnosis. The thermostat calls, the inducer motor spins up to prove draft through a pressure switch, the hot surface igniter glows, the gas valve opens, the burners light, the flame sensor confirms fire, and only then does the blower deliver heat to the house. In O'Fallon, MO, the failures cluster at predictable points: a flame sensor coated in oxide that can no longer prove the flame, a hot surface igniter cracked from years of thermal cycling, a pressure switch that will not close because the inducer is weak or the condensate trap on a high-efficiency model has clogged, or a control board with a burned relay. Each one stops the sequence cold, and each one mimics the others from the hallway.
When the AC quits in summer, see AC Repair.
Warning Signs for Furnace Repair
The sooner you notice these red flags, the easier and cheaper the fix. Watch for these telltale signs:
- Soot, rust, or moisture near the furnace cabinet or vent pipe
- Frequent on-off cycling that never reaches the thermostat setting
- A bang or boom when the burners ignite late
- A flame that burns soft and yellow rather than steady blue
- Squeals, scrapes, or grinding noises behind the blower door
- The blower runs constantly but the air never warms
- The furnace clicks and glows but the burners never light
- Burners light, run a few seconds, then shut down repeatedly
Act Before It Gets Worse
What seems like a minor issue today can take out a compressor or heat exchanger tomorrow. If you notice any of these signs, call (636) 200-4657 for a prompt inspection.
