If you drive a diesel, and you have problems starting the car when the engine is cold or when the temperature is low it's time to check the glow plugs. Besides other problems, this is the number one cause of poor starting with the diesel engine. Diesel engines have, unlike petrol engines, glow plugs that heat up the engine cylinder before the fuel injection. This is crucial for the ignition to happen, otherwise the fuel doesn't achieve the needed temperature for combustion.

Therefore, when your diesel car has problems with starting (cranking but not starting), in the morning or in cold temperature, this is the number one reason. Remember, one glow plug going off line is enough for your car not to start.

You'll find the glow plugs in the same position where the spark plugs would be (on the cylinder head). They will have a copper rail connecting them together and a thicker cable connected to the rail. The rail and the cable are responsible for the proper electric input to the glow plugs.

How to Check Does the Glow Plugs Working or Not

Before making any kind of replacement you first have to check whether the glow plugs are working or not. There are two ways you can perform this task:

1. While the plugs are in the cylinder and are disconnected from the input electric installation
2. Check them on the working table (which I strongly recommend).

The first option is to check the plugs while they are in the cylinder. You can do this by taking a piece of cable, connect it to the plus terminal on the battery and connect them to each separate plug. If there is a small spark when you touch the plug, that means that it's working. If not the plug is dead. However, in many cars the positioning of the plugs is very hard to reach so your at risk of MAKING A SHORT CIRCUIT while checking.(the test wire can easily touch the engine block). The plugs, in this case, must be separated from the main connection rail.

How to Take off Glow Plugs

In order to correctly check the plugs it would be best to take them out of the cylinder. This way you can make a 100% correct.
check. So, here's the procedure for taking the plugs off:

1. Make sure that ignition is turned off. If your not sure take the minus terminal of the battery.
2. Disconnect the main input cable (the one that comes from the installation to the rail).  
3. Disconnect the main rail connecting the plugs. Take off the screws that connect the rail to the glow plug. Do this one-by-one.
4. Take the proper wrench and take off the plugs. Do this one at a time. If the plug is going hard, take your time. Try spraying some anti-corrosion adhesive to the plug and wait. This is better than the plug breaking in two which leads to major problems (taking off the engine head).
5. Connect two cables, one going from the plus terminal of the battery, one from the minus terminal. Handle the plug with pliers as it will get hot. Put the minus cable to the metal body of the plug. The plus goes to the top of the plug, to the isolated contact.
6. When you connect the plug, there should be a heating of the top of the plug. If not, the plug is faulty. If the plug is OK, don't overdo with the connection or you will burn up the plug. Also, the plug should have equal heating all over it's top.

If you find that one or more plugs are not working, replace them all. In case of emergency you can replace a single one, but I strongly recommend changing all of them as soon as possible. For replacement of the plugs you can read "Changing the glow plugs" tutorial.