I have a 95 Altima. I tested the EGR solenoid valve by using a multimeter. Disconnect the two-wire connector that plugs into the solenoid valve. When engine is cold and ignition turned on the two wires from the harness should read 12volts. Momentarily touch the connector to the socket on the solenoid valve and you should hear an audible click as the solenoid valve snaps open and closed. Also you can check the continuity of the solenoid valve coil. It should show good continuity and not "open." If you can read ohms on your meter you can also check the coil and it should read about 50 ohms if it is good.
I was getting a P1400 code and thought my solenoid valve might be bad. It tested good so I removed the EGR valve and found it to be carboned up. A little choke cleaner fixed it up. Also the metal return tube that goes to the exhaust manifold was also carboned up. So far this has cured my problem.