In most diesel cars, there is a glow plug light and Check Engine light to help the driver determine if things are proper. The Glow Plug light will briefly light up, telling you the glow plugs are heating and turn off telling you to start the engine. The Check Engine light will appear when one of the sensors detects an out of spec or problem with the engine. Most engine check lights will flash if whatever condition is worsens or becomes more of a problem. The Check Engine light cannot report all things, for instance, if the water pump or fuel pump are bad or failing, it will not trigger an engine code warning you. You will just breakdown.
Many people do nothing when the Check Engine light appears on the panel. Like other sensors, you have to be able to read the codes to give you any idea of what is wrong. Some scanners can read codes "deeper", most readers that consumers buy cannot do this.
With a flashing glow plug light in a diesel, it means something has a major malfunction. Unlike the Check Engine light, which comes on when there is a malfunction and sends out an engine code, the Glow Plug light that flashes may or may not send out a code about what the malfunction is. If it does, then, the Check Engine light appears.
If the flashing glow plug light has NOT triggered the Check Engine light, there is a good chance it has nothing to with the engine. In a VW, the common problems that cause this condition are burnt out brake lights and the brake pedal switch, located behind the brake pedal. Before rushing to a VW mechanic that will charge $120 to hook the car up to the VW computer for codes, simply change the brake light bulbs, then, see if the light stops flashing and goes out. If if does not, depress the brake pedal a few times. In my case, after switching out the brake bulbs, the Glow Plug light continued to flash. Thinking it might be the brake pedal switch, I depressed the brake pedal a few times. The light went off. I suspect the plunger in the switch had got stuck, which caused the problem, in part. Both of my brake lights were bad.
Since my Check Engine light had not come on, I checked for codes- nothing. So, I knew it was not a serious malfunction as the VW manual stated it was. But, had the Glow Plug light not stopped flashing after depressing the brake pedal, I would have had to take it to VW, because there could be hundreds of other causes, like, bad connections, damaged wires to way more serious things.
If you do have a flashing Glow Plug light, before you go to VW, check the brake lights and brake pedal switch, especially, if the Check Engine light does not appear.