Glow plugs for diesel engines warm the engine block so that your diesel engine can begin running. If your glow plugs aren't working properly, you won't be going anywhere. Therefore, it's best to check and change them at proper intervals to keep you and your car motoring happily, not stranded by a roadside somewhere waiting for assistance.
If your engine refuses to start after turning over, chances are good that your glow plugs are no longer working properly. It's possible that you may get it to start after a few tries, but once you park the car and leave it alone for a few hours, you'll likely have the same trouble getting it to start again. Just like when the batteries in your TV remote are starting to die, this is your sign that you should check and possibly replace your glow plugs (see Reference 1).
What are They and Where are They Located?
A gasoline-powered vehicle relies on sparkplugs to ignite the fuel, causing the combustion that powers the engine. In diesel vehicles, the engine is powered by compression rather than combustion, but a cold engine doesn't work. Therefore, diesel engines use glow plugs (generally one per cylinder) to preheat the engine block before you start your car.
You should be able to find your glow plugs very easily, on one side and toward the top of your engine block (see Reference 2 for all information in this section).
Examination and Testing
Mark Cornwall of Carpartsdirect.co.uk advises that testing and replacing your glow plugs is an approximately 30-minute process, and can help avoid costly roadside assistance calls (see Reference 1).
Removing the glow plugs simply involves disengaging the wire leads attached to them, loosening the nuts holding them in place, and finally unscrewing them from your engine. Once you've done this, examine your plugs. If they're moist or blackened, you should probably replace them. It's easiest to replace all your glow plugs at the same time, so that you can easily keep track of when they were last changed.
If your plugs are greyish at the tips, test them to see if they're still working. Use a battery jumper pack and make sure you're not holding onto the glow plugs when you test them. One at a time, connect the positive cable from your battery pack to the top of a glow plug. Next, connect the negative cable to the main body shaft of your glow plug. If it's working properly, you should see the tip start to glow --- which is also why you don't want to be touching the plug when you're testing it. It's very simple --- if you don't see a glow, into the bin it goes. Purchase new glow plugs in most automotive supply stores, or order them online as well. Carpartsdirect.co.uk has produced a helpful video showing how to test your plugs; see References for a link.