Aprilaire digital humidifier control displays E5, and the humidifier will not energize the solenoid valve.

Safety first: Turn off power to the furnace and shut off the water supply to the humidifier before testing.

• Inspect solenoid wiring: Locate the Aprilaire solenoid valve on the water line. Check the two low-voltage wires from the control's H terminals to the solenoid. Look for pinched, cut, or melted insulation.
• Test solenoid coil: Disconnect one solenoid wire and measure resistance across the coil. Typical Aprilaire 24 VAC solenoids measure 20–60 Ω. A reading near 0 Ω indicates a shorted coil; OL indicates an open circuit.
• Check for shorts: Verify that the H wires are not connected to any 120 V source. They must only be on the 24 VAC control circuit.
• Replace failed parts: If the solenoid is shorted, replace the Aprilaire solenoid valve assembly. If the wiring and solenoid test good but E5 remains, the control’s output relay is likely damaged, and the Aprilaire digital humidifier control should be replaced.
• Retest: Restore power and water, then call for humidity and confirm that the solenoid energizes without errors.