A solenoid is a little known and less understood part of your engine that is nevertheless essential to the continued functioning of your vehicle. A solenoid is the part of the vehicle that works with the starter to get your engine running. The solenoid is essential to starting your engine, so it’s worth understanding how it works. The starting process takes place in two stages. If your car won’t start, then you might have a problem with the solenoid. See why it’s essential below.