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 250 — Primary limit switch open
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
The furnace is fine — that is the premise of this alert — so the cost question is really about whichever external device opened the interlock. The most common resident on a DS-style input is a condensate overflow switch, and when that is the trigger the underlying fix is drain clearing: cheap, often preventable, and covered inside a standard service call ($75–150). A damper end switch or field-installed safety that has failed is a small-parts repair on that device's side of the wiring. The spend to challenge is any furnace-side repair quoted against a 370: by Lennox's own definition the control is correctly reporting an external open circuit, so the diagnosis should march down the interlock wiring away from the furnace, and the ten-second continuous-voltage requirement means even an intermittent connection out there — not in the furnace — explains a stubborn or flickering alert.
DIY or call a pro
Start by identifying what is wired into the interlock, because that decides everything: if a float-style condensate overflow switch protects the furnace (common where a furnace or its coil sits over a drain pan), look for the visible version of its complaint — water in the auxiliary pan, a drain line that has stopped dripping outside, algae-clogged tubing. Seeing water tells you the interlock did its job, and clearing the drain path is on the maintenance end of homeowner work. What stays professional: tracing which device actually feeds the DS terminal when it is not obvious, and any rewiring — an interlock bypassed “temporarily” to restore heat silently removes whatever protection someone once thought necessary, and water damage or a stuck damper is exactly what it was guarding against. If no external device can be identified at all, that is the one version worth a technician promptly, since something opened a circuit nobody knew existed.
What this looks like
From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — that walkthrough covers what to check in order, and when it stops being a homeowner job.
This can also show up as the furnace not turning on at all — that page is a 5-minute checklist before calling anyone.