Equipment
- York gas furnace
- Cleaned and replaced flame sensor
- Verified gas flow
- Tested all safety controls
- Recalibrated ignition sequence
What the customer reported
A Dublin homeowner with a York gas furnace was getting inconsistent heat, the furnace would start, run for a short while, then shut itself off. Indoor temperature wouldn’t settle. The pattern was worse on colder days when the furnace was called to run longer cycles.
What we found
The diagnostic walked through the ignition sequence in order: gas valve opens, igniter heats, burner lights, flame sensor confirms flame, control board allows continued operation. The breakdown was at the flame-sensor step. The sensor was producing an unreliable signal, sometimes it confirmed flame, sometimes it didn’t. When the board didn’t get a clean flame-on signal within the safety timeout, it shut the gas off and locked out. That’s the “starts and then shuts down” pattern.
Flame sensors fail in a predictable way: combustion residue accumulates on the sensor rod over years of operation, eventually insulating the rod enough that the microamp current from the flame-rectification circuit drops below the board’s threshold. Cleaning often restores the signal; replacement is the durable fix.
What we did
- Pulled the flame sensor and cleaned it to baseline
- Tested the cleaned sensor, partial improvement but not back to spec, indicating sensor degradation beyond what cleaning could fix
- Replaced with a new model-matched flame sensor
- Verified gas flow at the valve and confirmed proper burner ignition pattern
- Tested all safety controls (pressure switch, high-limit switch, rollout sensors)
- Recalibrated the ignition sequence to manufacturer spec and ran multiple full heat cycles
The detail that mattered
Flame-sensor failures are one of the most common gas-furnace repair calls and one of the easiest to misdiagnose. The symptoms: short cycles, intermittent heating, look exactly like more expensive problems: gas valve, control board, pressure switch. Jumping straight to “replace the gas valve” without isolating the actual fault costs the homeowner money and doesn’t necessarily fix the problem. Diagnose first, then fix.
What the homeowner got
Furnace running clean, full heat cycles, consistent temperature, no more intermittent shutdowns. Safety controls verified. 1-year warranty on the repair.
The photos below show the ignition-system inspection during the diagnostic and the original internal condition of the furnace before service.
Photos
