A solenoid is an electrical device that is used to open or close valves or switches when current is passed through a component. It consists of thin wire looped around a metallic core. When electrical current passes through the solenoid, a magnetic field is produced. Solenoids have a variety of uses but the only drawback in using a solenoid is that it has only two positions, "fully on" or "fully off."
Check the physical condition of the solenoid. Be certain the wires and connectors are not broken or corroded. Check any fuses leading to the solenoid. Also check solenoid mounting. If not correct, it may result in reduction in power. Environmental issues such as excessive heat or corrosion will affect operation of solenoid. Correct any problems found during this check. If the solenoid still does not work, continue to Step 2.
Disconnect the power supply from the solenoid. Use a voltmeter/ohm meter (set to Volt) to check the power supply coming into solenoid. Do this by connecting one lead to the power supply and the other to a grounding source. Alternatively, disconnect solenoid from its component (starter, valve, etc.) and apply power. If working, the solenoid will magnetize -- which you can check with a screwdriver held gently across the solenoid. If the solenoid operates -- or magnetism is detected -- you are finished. If the power source is defective, troubleshoot and repair. Continue to Step 3.
Check that the solenoid is at or just above room temperature (72 degrees) and with the solenoid disconnected from the power source, take resistance readings. In the open or inactive position, the solenoid will have an ohm reading 0.2 ohms to 1.5 ohms -- depending on the size of solenoid. Check specifications for the solenoid being tested. Now check the closed or active resistance. The active resistance is much higher, ranging from 10 ohms to 110 ohms again depending on the solenoid. If you cannot remove the solenoid from the application, the ohm reading can be performed using the meter to measure across the wires in place -- without power. For the inactive measurement, place the meter leads between the white and black leads from the solenoid. For the active reading, measure between the red and black wires. If your solenoid does not have the specific wires as indicated here, it will be necessary to look at the wiring diagram -- located in the owner’s manual of the device being tested.
TL;DR (Too Long; Didn't Read)
Be sure that all connections and wires to the solenoid are functional. Invest in a solenoid tester that lights up when the solenoid is working properly.
Be sure that any power source is turned off or disconnected before testing circuits.