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 Gilroy, CA, 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.
Cooling trouble instead? See AC Repair.
Warning Signs for Furnace Repair
Recognizing the warning signs early can save you time and money. Keep an eye out for the following clues:
- Yellow, wavering burner flames where they should be sharp blue
- Short cycling every few minutes without reaching the setpoint
- Air moving through the vents that never gets warm
- A bang or boom when the burners ignite late
- Flames that start, last a few seconds, and go out again and again
- Rust streaks, soot, or water around the cabinet or flue
- The igniter glows and clicks, yet no flame ever appears
- Grinding or high-pitched squeals from the blower
Act Before It Gets Worse
A failing capacitor, clogged condensate drain, or dirty flame sensor rarely fixes itself and usually gets more expensive to address the longer it runs. At the first hint of a problem, call (408) 694-4630 for a fast inspection.
