Lennox E205 Error Code — What It Means
The E205 error code on a Lennox furnace means the control board has detected a gas valve control relay contact short. This is not a general gas valve failure code. Instead, the furnace’s integrated control is reporting a fault in the relay output circuit that drives the gas valve. Lennox’s technical documentation defines E205 specifically as a relay contact problem inside the control or in the wiring that connects the control to the gas valve.
In practical terms, the fault sits in the electrical pathway between the board and the valve, not in gas pressure or ignition components. The control board opens and closes a relay contact to energize the gas valve coil. When that contact or its circuit shorts, the board sets E205 and locks out the furnace to prevent unsafe operation.
Common Causes
- Damaged or shorted wiring harness Pinched, frayed, or melted insulation on the wires running from the control board to the gas valve can create a short circuit that triggers E205.
- Loose or corroded terminals Poor contact or corrosion at spade connectors on either the control board or the gas valve can cause intermittent shorts or mis-reads by the relay circuit.
- Miswired gas valve connections Crossed wires or incorrect terminal routing during installation or past service work can short the relay output and set the code.
- Failed control board relay contact The relay contact inside the integrated furnace control can weld shut or short internally, and Lennox specifies control replacement when wiring checks good.
Step-by-Step Fix
- Verify the E205 code at the furnace control display or LED fault history to confirm the active fault before you begin work.
- Turn off power to the furnace at the service switch and the circuit breaker to make sure safe inspection of wiring and control components.
- Inspect the gas valve wiring harness from the control board terminals to the gas valve terminals, looking for broken conductors, damaged insulation, pinched wires, corrosion, or any contact with chassis ground.
- Check all connections at both the control board output terminals and the gas valve spade or pin connectors, confirming each wire is seated firmly and routed correctly without crossing or shorting.
- Repair or replace damaged wiring if you find any defect in the harness or terminals, then restore power and retest to see if the fault clears.
- Replace the furnace control board if wiring is intact and properly connected, because Lennox’s procedure calls for control replacement when wiring checks out and the E205 persists.
- Restore power and clear the fault by cycling the furnace or pressing the reset button, then run a heat call to confirm normal ignition and operation without the error returning.
Parts Often Needed
| Part | Notes |
|---|---|
| Lennox integrated furnace control board | Amazon | Primary replacement when wiring is correct and the relay contact short remains. Match your model number exactly. |
| Gas valve wiring harness | Amazon | Replace if inspection reveals damaged insulation, broken wires, or corroded connectors that cannot be repaired. |
When to Call a Pro
Call a licensed HVAC technician if you are not comfortable working with 120 V furnace wiring or if you cannot identify obvious wiring damage. E205 involves both live control circuits and the gas valve circuit, so misdiagnosis or improper repair can create a safety hazard. A technician has the meters and Lennox service literature to trace relay outputs, confirm proper voltage at the gas valve, and install the correct OEM control board for your furnace model. If the fault persists after a control replacement, the technician can also check for rare issues such as a gas valve coil drawing excessive current or a grounding fault elsewhere in the unit.