A sprinkler solenoid is an electromagnet that uses current from the controller to open and close the valve. The electromagnet operates a plunger that changes pressure in the valve as it moves up and down. Solenoids wear out over time and prevent the plunger from sliding in and out smoothly. Before you change out a sprinkler solenoid, determine which type of valve you have and purchase a replacement of the same type.
Turn off the water supply. Find the backflow preventer. It should be somewhere between the water meter and the irrigation controller. Grasp one of the two valve handles and crank it 1/4 turn to a vertical position. Turn the controller to the "off" position.
Remove the round valve box cover by turning it counterclockwise and pulling up. Locate the solenoid. It will be shaped like a cylinder, often with one flat side, and two wires connected to the top. Grasp the solenoid and turn it counterclockwise to unscrew it. You should see a silver plunger at the bottom. If the plunger falls out or is missing, replace the solenoid.
Push on the plunger, if it's present, a few times with your finger. It is spring-loaded and should pop back out smoothly. If not, replace the solenoid.
Disconnect the two wires. Turn the wire nuts counterclockwise to unscrew them and set them aside. If necessary, untwist the wires once exposed, and discard the old solenoid.
Clean the exposed ends of the wires that lead to the controller with a towel. If they are frayed or damaged, cut about ½ inch off with wire cutters, and remove about 1 inch of the insulation. Separate the wires connected to the new solenoid.
Insert the exposed end of one of the controller wires into a new wire nut. Insert the exposed end of one of the solenoid wires into the same wire nut. Grasp the two wires in one hand, and screw the wire nut clockwise a few turns until it fits firmly. Repeat this process to connect the other two wires.
Connect the new solenoid to the valve. Gently insert the solenoid with the plunger side down into the slot. Turn clockwise until it fits snugly. Do not overtighten the solenoid, as this may cause the valve to crack. Gather the excess wire slack and set it inside the valve box. Replace the valve box cover and turn it clockwise slightly to tighten.
Test the valve. Go to the backflow preventer and locate the handle you previously turned off. Crank it ¼ turn until it is in a horizontal position . Turn the controller to the "on" position. Turn on the station for the appropriate valve.