Furnace display shows E17; the unit may run but with reduced capacity or may lock out in extreme temperatures.

Safety first: Turn off power before working on sensor wiring.

• 1. Locate outdoor sensor: Find the Thermia outdoor temperature sensor mounted on an exterior wall, away from direct sunlight and exhaust vents.
• 2. Inspect wiring: Check the cable from the sensor back to the furnace control board for cuts, abrasions, or loose connections.
• 3. Test sensor resistance: Disconnect the sensor and measure resistance with a multimeter. Compare to Thermia's temperature/resistance chart for the current outdoor temperature. Replace the sensor if readings are significantly off.
• 4. Check placement: If the sensor is in direct sun or near a dryer or furnace exhaust, relocate it to a shaded, representative location.
• 5. Reset and test: Restore power and verify that E17 clears and that outdoor temperature readings are reasonable.