Lennox E370 Error Code

What does Lennox E370 mean?

E370 is a Service Urgent alert on Lennox communicating furnaces (EL296, SL280, SLP99UHV series) indicating that the DS-R interlock terminal has lost 24VAC for two consecutive minutes. In communicating mode the DS-R terminal (jumper W914 cut) monitors a field-installed interlock device such as a float switch or condensate overflow switch; when the control sees no voltage at DS for two minutes it disables all heating outputs and sends alert E370. The alert clears automatically when 24VAC is sensed continuously on the DS terminal for at least 10 seconds, or on a power reset.

Symptoms

• Furnace stops heating and blower de-energizes; thermostat displays 'GF Interlock Switch Open' or alert code E370.
• All heating outputs are disabled for the duration of the alert; the furnace will not attempt ignition.
• If an existing interlock switch alert is present at power-up, the control re-sends the alert until 24VAC is restored to DS.
• Alert clears without manual intervention once the interlock circuit is re-established and 24VAC is sensed on DS for 10 or more seconds.

Common causes

• Float switch in the condensate drain pan has tripped due to a blocked or slow-draining condensate line.
• Field-installed interlock switch (e.g. air handler drain pan overflow switch) wired to DS-R has opened.
• Blower door or access panel interlock switch is not making contact — door not fully closed or switch arm broken.
• Wiring fault: loose connection or broken wire on the DS terminal circuit interrupting 24VAC.
• In non-communicating mode: DS-R jumper (W914) not cut, causing the thermostat to detect loss of power to DS when the link is absent.

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Diagnostic steps

1. Identify and inspect the interlock device wired to DS-R

   Locate the device connected to the DS-R terminal on the furnace control board — typically a condensate float switch or drain pan overflow sensor. Inspect for water in the pan, a tripped float, or a visibly open switch. Check that all access panels are fully seated so the door switch makes contact.

2. Clear the condensate system

   If a float switch has tripped, the condensate drain is likely blocked. Remove and flush the condensate trap with clean water. Verify the drain line flows freely to the drain point. On SLP99UHV and similar condensing furnaces, also clear the cold-end heater box drain strainer.

3. Measure 24VAC at the DS terminal

   With power on and the interlock device in its normal (closed) state, measure voltage between DS and C on the furnace terminal block. The reading should be 24VAC. A reading of 0VAC with the device closed confirms a wiring fault or open connection in the DS circuit.

4. Inspect DS-R wiring and connections

   Trace the wiring from the DS terminal to the interlock device and back to R. Check for loose screw terminals, damaged wire insulation, and corroded connectors. Re-terminate any suspect connection and re-measure voltage at DS.

5. Verify W914 jumper state (non-communicating mode only)

   In non-communicating applications where W914 is NOT cut, the DS terminal is fed through the thermostat circuit. If E370 appears in this configuration, check that the thermostat is powered and the R-to-DS path is intact. Per Lennox documentation, in communicating mode W914 must be cut and the DS terminal monitored independently.

6. Confirm alert clears and system restores heating

   After correcting the root cause, restore power or wait for the control to detect 24VAC on DS for 10 seconds. Confirm E370 no longer appears on the thermostat and that the furnace completes a full heating cycle without re-triggering the alert.

When to call a professional

Call a licensed technician if the condensate line remains blocked after clearing the trap, if the DS circuit cannot be traced to a specific interlock device, or if 24VAC is absent at DS despite the interlock switch appearing closed. On communicating systems the W914 jumper configuration and DS terminal wiring must comply exactly with the Lennox wiring diagram for the specific model; incorrect wiring can cause persistent E370 alerts that require factory-level documentation to resolve correctly.