Dettson furnace displays E13; the unit may short-cycle or not reach the set temperature.

Safety first: Turn off furnace power before checking outdoor wiring.

• 1. Locate the outdoor sensor: Typically mounted on an exterior wall away from direct sunlight and exhaust vents.
• 2. Inspect cable: Follow the sensor cable back toward the furnace. Look for damage from weather, UV, or animals. Repair or replace as needed.
• 3. Check connections at the control board: Ensure the sensor wires are landed on the correct terminals and are tight.
• 4. Test sensor: Disconnect and measure resistance; compare to Dettson's temperature/resistance chart. Replace if out of range.
• 5. Clear error: Restore power and verify that E13 clears and the furnace modulates or stages correctly.