I have a 1990 Mustang 5.0 and there's a scanner error code 81 what does this mean?

Code 81 †Secondary Air Injection Diverter Solenoid failure AM2. The secondary air diverter solenoid valve isn’t working on the Thermactor Air System (smog pump). Look for broken or missing vacuum lines on the solenoid valve to the diverter valve. The solenoid valve is located on the back side of the passenger side wheel well. Check the solenoid valve for +12 volts at the Red wire and look for the Lt Green/Black wire to switch from +12 volts to 1 volt or less. The computer controls the valve by providing a ground path on the LT Green/Black wire for the solenoid valve.