To troubleshoot solenoids, first use a voltmeter to gauge the electrical signal and determine if the current is correct. Next, inspect the solenoid for loose or missing covers. Find the push rod and check if the pin is the right length to actuate the valve. Check the springs above the diaphragm of the valve. Finally, check if the proper current is running to the solenoid.
If the electrical signal to the solenoid is not strong enough to meet the AC current, the coil is likely burned out and needs to be replaced. If the solenoid covers are loose or missing, check for small pieces of metal inside the solenoid. The covers prevent debris from interfering with the magnetic field, so any foreign metal needs to be cleaned away. Secure the covers completely.
If the pin on the push rod is no longer the right length due to wear over time, replace it with a pin from the same manufacturer as the original. If the spring is faulty, the diaphragm cannot open or close correctly, causing a water flow issue. Replace or adjust the spring, if needed. When checking the current running to the solenoid, reference the manufacturer's voltage requirements. Make any necessary adjustments.