The glow plugs on the Shalaxy TDI/PD diesel engines are not a dual coil device. They are simply heated tips which take about 10 amps each and two or three seconds to reach red hot temperature. When you turn the ignition key, the ECU works out from the engine and ambient temperature whether to energise the glowplugs and for how long - they are either on of off. The tips are situated very close to the injector spray pattern. In cold weather, and with a cold engine, particularly if the engine cranks slowly, the compression stroke may not heat up the cylinder air charge sufficiently to ignite the diesel. The red hot glow plugs ensure that ignition occurs in the vicinty of the plugs at least, once the engine starts the chamber temperatures rise quickly and after a few seconds, the ECU will turn the glow plugs off. The ECU cannot check whether the glow plugs are working or not - they operate via a simple relay. The glow plug light is the ECU's advice for you to wait until it think the temperatures are hot enough for a trouble free start. The glow plugs do continue to operate for 3 or 4 seconds after the light has gone out. Symptoms of a faulty glowplug are difficult starting from cold with increased smoke and rough running for a few seconds after starting. If you don't wait long enough you might experience rough or lumpy running for a few seconds. AFAIK, at temperatures of 20 deg plus ambient or if the engine is warm, the glow plugs do not activate at all. It only takes one or two faulty glow plugs to make starting in cold weather a pig. Unfortunately, like spark plugs, glow plugs tend to have a limited life and they are worth checking after about 30,000 miles. To check the glow plugs you need to remove the engine cover, and pull the busbar rail off the glow plugs (this require a hefty tug). Check each for resistance to the engine block. They are usually 0.15 ohm to 0.3 ohm. Faulty plugs are usually open circuit or several hundred ohms plus. If you don't have a multimeter - use a 12v bulb (6w to 21W) - if it lights via a plug (other end to +12v), the plug is OK. Avoid cheap glowplugs - they dont last long - sometimes only a matter of weeks. I recommend NGK, Beru or Bosch. Use a good motor factor or major company like GSF,Eurocarparts or www.vwspares.co.uk, they will be less than half the price of a dealer and will be the genuine item unlike some Ebay bargains.