Volkswagen (VW) Diesel Starting Problems and Diagnosis
While this information can apply, generally, to many cars, it is specifically for Volkswagen diesel models known as TDI. These are usually the Jetta or Passat models.
Diesel engines are high pressure engines and have no spark plugs, but they do have something similar called glow plugs. They are usually found in similar positions like spark plugs are on the engine. A harness attaches to them.
If you get into your car nothing happens, suspect the battery is dead. There may be enough power to turn on the radio or other small devices. Usually the headlights will not come on. Assuming the battery is fine, if the car cranks over and over but will not start, think fuel pump and filter. If the engine turns over but will not start, insufficient fuel is reaching it. These pumps will either intermittently work or simply stop. There is never a warning and it will not cause the engine light to come on. If your car simply clicks when you try to start, think the starter has failed. All cars need a starter. Again, this will not cause the engine light to come on and suddenly stops, there is never a warning when a starter fails.
In the VW, a bad or failing glow plug will cause the engine light to come on. A VW can still run with one bad glow plug, if two are bad, starting will be very hard and you will wear down your battery. The harness attached to the plugs can also cause the engine light to turn on. When testing glow plugs, the results using an Ohm meter should be .5 to 1.5 ohms AND all plugs should be within .3 ohms within of each other. If not, one of the plugs is bad or going bad. If the plugs are fine, check the harness to determine if the 12volts is getting to the the plugs. Corrosion and age can cause problems. If the test read out is high in ohms, the harness is bad.
Of course, if you can read the engine codes, a bad plug or harness will throw a code. If you car starts but idles rough and jerky at times, it could be the injector. A misfiring injector will throw a code also and tell you which cylinder. If the injector is bad or needs adjustment, unless you know how, this is not for a DIY project because it requires the VW computer used at dealers. Replacing injectors are expensive, a new one runs $800, while a rebuilt one, is less.