The EGR valve is vacuum controlled. The ECU sends a signal to the EGR control solenoid valve that lifts the EGR valve to a certain position. The EGR control solenoid has its vacuum regulated by the vacuum control solenoid.

A code 12 is set when the lift sensor in the EGR valve does not reach the height when the ECU opened the control solenoid valve. The problem can be the EGR valve, the vacuum hoses, the EGR control solenoid, the EGR vacuum control solenoid, the ECU, or the wiring.

If you look in that shop manual, page 11-115 starts the diagnosis for a code 12. You should try to go through those steps to see what is going on.