All you will need for this method is a multimeter and this guide. Without this method people generally start by replacing the first bulb and then move on to replacing each bulb in turn until they find a bad one.
This is very time consuming and very frustrating! If there ever was a reciperecipe for procrastination, this is it! You will be glad to know that this method is much quicker and all you need is a multimeter, a little time, and a steaming mug of coffee.
A Step By Step Process
The first step is to check that the fuse in the plug is intact. Take the fuse out of the plug and test with your multimeter. You can find an article on how to use a multimeter here.
The next step is to check the first and last bulb to make sure that they are working as designed.
To do this, remove the bulb from its holder and identify the two contacts. Connect the two points of the multimeter to the two contacts on the bulb and you should hear a buzz from the multimeter.
This confirms that the bulb is OK. If at any stage of this process you find that a bulb is not working, you can replace it and you will be finished.
Presuming the first and last bulb are working fine you can proceed to the second step. You need to look at the socket from where you removed the first bulb. You will see three wires.
One that came from the plug (or previous socket), one that is going to the next socket, and one that bypasses the socket. At the final socket there will only be two wires.
These are the one that came from the previous socket and the other wire that bypassed the other light sockets and is going back to the plug. At the very first socket connect the probe of the multimeter to the contact of the wire going to the next socket.
At the last socket connect the probe to the contact of the wire coming from the previous socket. The multimeter should not buzz; it should not indicate continuity. This confirms that the faulty bulb is between the first and the last socket.
This Troubleshooting Method is so Much Quicker!
Usually people would check each bulb in turn but with this method we will divide the number of lights we are testing in two distinct sections each time we do a test. Remove the middle bulb of the string and test it with the multimeter. Then connect the first probe of the multimeter to the first socket in the string in the same way as in the last test.
Then connect the second probe to the socket in the middle of the string you just removed the bulb from. Connect it to the contact of the wire coming from the previous socket.
If the multimeter buzzes you know that the first half of your string of lights is working as designed, so the problem is obviously in the second half of the string. If it doesnt buzz you know you need to concentrate on the second half.
If you had a string of 400 bulbs, you now know that the broken bulb is one of either the first 200 or the last 200, so you have halved the number of bulbs to be tested.
If you keep following this process you will reduce the number of potentially damaged bulbs to 100, 50, 25 and so on until you find the damaged bulb. This is a lot quicker than testing each of the 400 bulbs in turn! There is power in working methodically.
What If Two Bulbs Are Burned Out?
It is entirely possible, in some cases probable with older strings, that two bulbs could be gone. To identify this situation you can test both halves when you divide the string in two. For example, when you initially selected the middle bulb, you can test its connection to the first bulb and to the last bulb.
If the multimeter fails to buzz on both sides then you know there is a faulty bulb on both sides. You then need to divide again and test both sides of the new divide. When you test a section where you do get a positive result from the multimeter you know that all the bulbs in that section are working ok.