If your car won't start or you're noticing a drop in performance, then it's possible that the culprit is a faulty distributor cap. The distributor cap ultimately sends the ignition signal to each of the spark plugs, which in turn help ignite the engine. Thus, if the distributor cap isn't working properly it's likely something that you'll notice right away.
Checking the distributor cap is something that most car owners should be able to do, and it should be pretty easy to diagnose it as the culprit in case of ignition trouble. However, checking this part means unplugging all of the spark plugs. The plugs should be easy to spot, as they are all connected to the cap itself.
Whenever you're dealing with spark plugs, keep in mind that each plug is assigned to a specific outlet on the distributor cap. One mistake that many drivers make is thinking that spark plugs are interchangeable, and as long as all of them are plugged in somewhere they're good to go. This is not the case. That's why it pays to mark each plug with a number, and write that number on the outlet that it's plugged into as well. This makes reattaching the spark plugs a breeze, because you don't need to worry about what went where. Doing this once will also pay off for the future with any job that requires disconnecting the spark plugs.
Once the wires are taken care of, you can get at the cap itself. Use a screwdriver to remove the cap and check underneath it. Specifically, you should be looking for the electrodes or contacts, as these send the signal to the spark plugs. If one of these looks oxidized or burned, you'll likely be needing a new distributor cap. Check around the interior of the cap as well for any black streaks. This usually indicates that the spark plugs are misfiring when they start. In this case, you'll not only need a new cap, but a new set of spark plugs to boot.
One last possible issue with the distributor cap typically leaves no visible clues. In this case, you'll need an ohmmeter to diagnose the problem. You could turn the issue over to a mechanic, but many auto parts stores also sell these devices, and they're handy to have for a wide range of automotive and home repair issues.
Take one lead of the ohmmeter and attach it to the exterior of the cap, where the ignition coil connects. The other lead should be placed inside the cap, where the coil comes through the top. You ideally want your reading to be zero, or very close to it. Ohmmeters measure resistance, so a high resistance simply means that signals are not transferring as well as they could be, which results in decreased performance.
If you do end up needing to replace a distributor cap, it's fortunately not a major repair. You've already removed the original cap, so putting a new one on is as simple as buying the part and attaching it much in the same way you would attach the old one. Before doing so, compare the two caps and make sure you mark the new spots where the plugs will go with the correct corresponding numbers. This way, you can easily attach the spark plug wires you marked earlier.
If you've done everything right, your engine should roar back to life when you start it, and if the car was dealing with weak performance before, you should see a noticeable improvement.