Glow Plug Indicator Light
Unlike cars with gasoline engines which can start when the engine is cold, diesel engines must be warmed prior to starting. Specifically, there must be heat in the cylinders prior to starting the engine. Once you turn the ignition on a diesel vehicle with a cold engine, you should see this light come on in. It means the glowplugs are operating to warm the engine. It will remain on until the engine is warm enough to start.
If the engine is cold and struggling to start, but the glow plug indicator light does not come on when the ignition is first switch on, then it is possible there could be a problem with the glow plug system.
On some manufacturer's systems this indicator light has other additional functions. If it is permanently lit up or flashing, it could mean there is a diesel fuel injection fault. Alternatively, on some VAG vehicles if the glow plug indicator light is lit in conjunction with the diesel particulate filter warning lamp, it indicates the condition of the filter.