Aprilaire digital humidifier control displays E1 and humidifier will not run

Safety first: Turn off power to the furnace/air handler at the service switch or breaker before working on any Aprilaire humidifier wiring.

• Verify sensor location: Locate the Aprilaire outdoor temperature sensor (usually mounted on an exterior wall, wired to the ODT/OD terminals on the digital humidifier control).
• Inspect wiring: Check the two-conductor cable from the sensor to the control. Look for cuts, staples through the cable, rodent damage, or pulled-out conductors. Reseat the wires firmly under the ODT/OD screws.
• Check for shorts: Make sure the two sensor wires are not touching each other or any metal. Separate and re-terminate if needed.
• Test the sensor: With power off, disconnect the two sensor wires at the control and measure resistance across the sensor leads with a multimeter. Compare to Aprilaire’s temperature/resistance chart (typically several kΩ at room temperature). If reading is open (OL) or near 0 Ω, the sensor is defective.
• Bypass for test: Temporarily install a known-good Aprilaire outdoor temperature sensor or a resistor of appropriate value across the ODT terminals to see if the E1 clears when power is restored.
• Replace if bad: If wiring is intact and readings are out of spec, replace the Aprilaire outdoor temperature sensor and clear the error by cycling power.