Lennox Furnace Code 13
WarningShown as: 13 rapid flashes (1/2 second on, 1/2 second off), then the LED stays off for 3 seconds and repeats
The 120-volt supply to the furnace is below the acceptable range.
Manual wording: “13 - Low Line Voltage”
Documented for: ML180UHE, ML193UHE and other Lennox furnaces using SureLight integrated control 103217-03 or 107163-01
Likely causes, most common first
- Utility supply voltage low or a brownout condition
- Undersized or loose branch circuit wiring feeding the furnace
- Loose connection at the disconnect or junction box
What you can safely check yourself
- Check whether other appliances and lights in the house are also acting up, which suggests a house-wide supply problem
- Check that the breaker feeding the furnace is fully on
When to call a technician
Measuring and correcting supply voltage is electrical work. Have an electrician or HVAC technician verify the voltage at the unit.
A Lennox furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Installation Instructions ML180UHE (507103-04, 04/2024) - Integrated Control 103217-03 and 107163-01 RED LED Flash Code tables (Lennox Industries Inc., ML180UHE series 80% AFUE upflow/horizontal gas furnace) · checked 2026-08-25
- Installation Instructions ML193UHE (507863-02, 06/2020) - Table 10, RED LED Flash Code / Diagnostic Codes / Status of Furnace (Lennox Industries Inc., ML193UHE series 93% AFUE upflow/horizontal gas furnace) · checked 2026-08-25