How to quickly fix E3 error on Aprilaire 500 whole house humidifier. 1 - try simple reset 2 - check resistance 3 - check outdoor unit and wiring 4 - if wiring looks good and your getting zero resistance, then replace outdoor unit. ( 10 bucks) 5 - If you replace outdoor unit and still have issues you may need to run new wire to outdoor unit or need new control panel. Resistor Table – You need this table along with the outdoor temperature to determine what OHMs you should be reading. So if it's 30