HVAC Code Lookup

Lennox Furnace Code 4

WarningShown as: 4 rapid flashes (1/2 second on, 1/2 second off), then the LED stays off for 3 seconds and repeats

The flame signal the control reads back from the flame sensor is too weak. The burner may still be lighting, but the control is warning that the flame-proving signal is marginal.

Manual wording: “4 - Low Flame Signal - check flame sensor”

Documented for: ML180UHE, ML193UHE and other Lennox furnaces using SureLight integrated control 103217-03 or 107163-01

Likely causes, most common first

  1. Flame sensor rod coated with oxide or scale
  2. Poor unit ground, which lowers the measurable flame current
  3. Burner or manifold problem producing a weak or lifting flame

What you can safely check yourself

  • Check and replace the air filter, since poor airflow can distort the flame
  • Turn the furnace power switch off for 30 seconds and back on once

When to call a technician

Cleaning or replacing the flame sensor and measuring flame current in microamps requires opening the burner compartment and using a meter. That is a technician job.

A Lennox furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Other Lennox codes