Unlike gasoline engines, which use a spark to ignite the fuel mixture, diesel engines rely solely on cylinder pressures to ignite the diesel fuel mixture. If there are problems with your glow plugs, it is important to get them replaced as it will affect how your vehicle starts. On cold wintery mornings, fully functioning glow plugs are essential for a successful ignition. In case your glow plugs fail and you get stuck in the cold with an engine that will not start, make sure you keep a blanket in the boot of your car.