Furnace Flame Sensor Guide: The #1 Cause of No-Heat Calls in Minnesota

Furnace Flame Sensor Guide: The #1 Cause of No-Heat Calls in Minnesota

The flame sensor is a small rod that verifies your burner actually ignited. It's inexpensive—often $5–$15 as a part—yet a dirty or failed flame sensor is the single most common cause of furnace no-heat service calls in Minnesota each winter.

How the Flame Sensor Works

After the gas valve opens and the burner lights, the flame sensor detects the flame through a process called flame rectification. The flame completes an electrical circuit between the sensor rod and the burner ground, producing a small microamp current the control board reads. If the board doesn't detect this current within 7–10 seconds, it shuts off the gas valve as a safety measure.

Symptoms of a Dirty or Failed Flame Sensor

Furnace lights briefly (1–7 seconds) then shuts off. After 3 failed attempts, furnace locks out and won't try again until reset. Furnace works intermittently—lights sometimes, shuts off other times. This pattern is almost always a dirty flame sensor rather than a failed one.

Cleaning vs. Replacing the Flame Sensor

A dirty flame sensor can often be cleaned by a homeowner or technician: shut off power, remove the sensor, lightly sand the rod with fine steel wool or emery cloth, reinstall. This $0 fix resolves most flame sensor issues. If cleaning doesn't help, replacement sensors cost $15–$40 and install in minutes.

When to Call a Pro

If cleaning the flame sensor doesn't resolve repeated lockouts, the issue may be with the gas valve, control board, or pressure switch. See our control board guide, gas valve guide, and emergency guide. Furnace Direct carries Goodman replacement parts.