Lennox thermostat shows E351 outdoor temperature sensor error; system may still cool but displays incorrect outdoor temperature or uses wrong staging.

Safety: Turn off power to the outdoor unit before accessing the control compartment.

• 1. Locate the sensor: On many Lennox condensers, the outdoor temperature sensor is a small probe or thermistor mounted near the coil or in the control compartment.
• 2. Inspect wiring: Check the sensor leads for cuts, corrosion, or loose connections at the control board. Repair or re-terminate any damaged wires.
• 3. Check sensor placement: Ensure the sensor is not in direct sunlight or touching hot discharge lines, which can cause inaccurate readings. Reposition according to Lennox guidelines (typically shaded, free-air location).
• 4. Replace sensor if defective: If the sensor is visibly damaged or readings are obviously wrong (e.g., 150°F on a mild day), replace with the correct Lennox outdoor temperature sensor part.

Note: While this fault may not stop cooling, it can affect efficiency and staging. If replacing the sensor does not clear the error, the control board may be at fault.