Aprilaire digital humidifier control shows E6 and humidifier does not run when furnace is on

Safety first: Turn off power to the furnace before opening the blower compartment or accessing wiring.

• Verify interlock method: Determine if the Aprilaire humidifier is wired to the furnace W (heat call), G (fan), a sail switch, or a current sensing relay. Refer to the installation diagram on the furnace or in the Aprilaire manual.
• Check wiring continuity: Inspect the two wires from the humidifier control to the proving device (sail switch or current sensing relay). Tighten any loose connections and repair damaged cable.
• Test sail switch: With power off, disconnect one wire from the sail switch and measure continuity across the switch while manually moving the sail. It should close (0 Ω) when airflow is present. Replace if it does not change state.
• Test current sensing relay: Confirm the relay is clamped around the correct blower motor lead and that its contacts close when the blower runs.
• Correct furnace terminal wiring: If wired directly to W or G, ensure the control is on the correct terminals and that the furnace board is providing 24 VAC when there is a call for heat/fan.
• Replace failed device: Replace the Aprilaire sail switch or current sensing relay if they do not close under airflow.