Lennox E205

What does Lennox E205 mean?

E205 indicates the IFC has detected a shorted contact on the gas valve control relay inside the board. The relay should open when the gas valve is de-energized; a shorted contact keeps the valve circuit energized even when the board commands it off. This is a safety-critical fault — a stuck-closed relay can allow unintended gas flow — and the board logs E205 and shuts down the heating sequence.

Symptoms

• E205 alert on thermostat or control board with furnace shutdown.
• Gas valve may remain energized after the thermostat heat call ends.
• Furnace attempts to ignite but shuts down before completing the sequence.
• Repeated attempts to start result in immediate E205 lockout.

Common causes

• Failed relay contact inside the IFC board welded closed by an arc during a prior gas valve fault.
• Incorrect wiring on the gas valve causing sustained overcurrent that welded the relay.
• Manufacturing defect or end-of-life failure of the relay on the control board.
• Surge voltage event damaging relay contact surfaces.

Diagnostic steps

1. 01

   Confirm correct gas valve wiring before assuming board failure

   Check gas valve wiring per the SL280DFNV wiring diagram. A mis-wired valve can cause abnormal current through the relay that ultimately welds the contact. Correct any wiring errors first.

2. 02

   Measure gas valve circuit voltage with board powered

   With the heat call off, measure voltage across the gas valve solenoid terminals. If 24V is present when the board should have the valve de-energized, the relay contact is stuck closed and board replacement is required.

3. 03

   Verify gas valve solenoid resistance

   Disconnect the valve leads and measure solenoid resistance. An open or very low resistance solenoid may have contributed to relay failure by drawing abnormal current; replace the valve along with the board if so.

4. 04

   Replace the IFC board

   E205 caused by a shorted relay contact requires IFC board replacement. Install the Lennox-specified replacement board, re-enter configuration parameters, and verify gas valve circuit operation.

5. 05

   Test through multiple heat cycles after replacement

   Run the furnace through at least three complete heat cycles. Verify E205 does not return and gas valve de-energizes properly at the end of each call for heat.

When to call a professional

A shorted gas valve relay is a safety-critical failure that can cause unintended gas flow. Shut off the furnace at the gas shutoff valve if you suspect the valve is remaining open when it should be closed, and call a licensed technician immediately. IFC board replacement also requires correct capacity code reprogramming on communicating models — do not operate the furnace until the fault is fully resolved.