Lennox furnace Alert code 370: Interlock switch open — no 24 volts at the DS terminal
Alert 370 means the control is not sensing 24 volts AC on its DS terminal, which is the interlock input. It clears when that voltage is present continuously for at least ten seconds.
Applies to: Lennox communicating gas furnaces with both variable speed and constant torque blower controls, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
The DS terminal is an interlock input: an external device is wired into it and holds 24 volts there while it is safe for the furnace to run. Anything wired to interrupt that voltage — a condensate overflow switch, an external safety, a damper end switch, a field-installed interlock — will produce this alert when it opens. The alert is therefore not a fault in the furnace at all. It is the furnace correctly reporting that something else told it to stop. The useful detail in Lennox's own note is the timing: the alert clears only after 24 volts is sensed continuously for a minimum of ten seconds, so a marginal or intermittent connection will produce an alert that will not clear even though voltage appears to be present.
Most common causes
- A condensate overflow switch that has tripped because the drain is blocked, which is the most common wiring at this terminal.
- An external safety device or interlock wired into the DS terminal having opened.
- A damper or ventilation end switch not making.
- A loose, corroded or broken connection at the DS terminal itself.
- A failed 24 volt supply to whatever device feeds the terminal.
How to diagnose it
- Check the condensate drain and the area under the furnace for standing water, since a tripped overflow switch is the usual explanation and a wet floor is visible without opening anything.
- Note what else was installed with the furnace — a humidifier, a ventilator, a zoning system — since any of them may be wired to this input.
- Note whether the alert clears briefly and returns, which points at an intermittent connection rather than a tripped safety.
- Do not jumper the terminal to clear it. Whatever is wired there was wired there deliberately.
How to fix it
- If the cause is a blocked condensate drain, clearing the drain and letting the overflow switch reset resolves it.
- Otherwise this is technician work to trace what is wired to the DS terminal and why it opened.
- Lennox notes the alert is sent on a power reset if voltage is not sensed, so a reset will not hide it.
- The monitoring applies to both variable speed and constant torque controls, so the alert is not specific to one blower type.
Notes
Because the interlock is a field connection rather than a factory one, what it is wired to varies between installations. Two furnaces of the same model can show this alert for entirely different reasons.
Related
- Alert code 276 — Furnace calibration failure, or high pressure switch fault
- Alert code 251 — Primary limit switch open
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.