If you want to solve any kind of electrical problem on your car you must know how to use a 12V circuit tester.

This simple and cheap tool is easy to use yet very effective in finding a problem.

It may be somewhat obsolete due to modern technology in cars, but it still is valid, especially in a combination with modern OBD tools.

In this article, we’ll show you how to properly use it. At the end of the article, you can find a video tutorial that you can also watch on our YouTube channel.



The name of the tool pretty much says it all but nevertheless, here’s the basic definition:

A circuit tester is a tool that enables you to make a very simple diagnostic of a problem on your car’s electric installation.

It consists of a probe with a light bulb inside of it on one side and a metal clamp on the other. These two are connected with a cable.

There are differences in the kind of probes, cables, and clamps used but the basic principle is the same.

The probe may be more like a screwdriver or a pick, the cable may be shorter or longer, and the clamp may be bigger or smaller.

Also, the one we’re showing is for a 12-volt installation but the basic principle is the same for a 24-volt tester which can be used on bigger pickup trucks and trucks.




The most common principle of testing any electric installation is checking several reference points if there’s an electric current or not.

The circuit tester does exactly this. By connecting it to the electric installation in some way and letting current through it, it gives you insight into what and where the problem may be.

When there’s an electrical current, the light bulb flashes up, when there isn’t there’s no light.

So, this simple tool turns electric current into a clear visual sign that should help you determine the problem.



Using a 12V circuit tester is very simple. But have in mind that you can test two kinds of connections:

You can test a plus (12 volts) connection and a minus or ground connection.

The principle is the same only the connection is different.

For checking a plus, 12-volt connection:

  • Connect the metal clamp to a bare, metal surface with a good connection to the minus (ground) connection. It’s even better if you can connect directly to the minus battery terminal.


It can be a screw, a piece of bare metal, or else. If you’re checking around the engine bay, just find a bare screw, nut washer, or else. The best situation is when the clamp can reach the minus battery terminal.

If you’re doing this inside the passenger cabin, like when checking the fuses, for instance, avoid insulated metal parts that may have rubber, plastic, or else on them.

  • Use the probe for inspecting the electric connection


You can use the probe in two basic ways: as a tester and just touch the bare connection or use it to penetrate the wire insulation.

When penetrating the insulation, be careful not to cut or damage the cable or cause an accidental short circuit.

Just hold the cable and slightly puncture the insulation. If you’ve done it properly, nothing but a small, barely visible dot will be left on the cable.

Now, if you want to use a 12V circuit tester to check a ground connection: do exactly everything the same just swap the connections.

Connect the clamp to positive, 12-volt input and use the probe to check the ground connection.


What’s important to know on this part is  If the cable you’re testing is showing a good ground connection, the light bulb will shine brightly.

But if it’s weak, the light bulb will be dim or barely visible. This means there is an existing but weak ground connection.

More on-ground connection problems in this article.

So, to simplify a bit on this part: connect the clamp to one side and poke with the probe on the other side. When the bulb lights up, you have current, when it doesn’t you don’t. This is why it’s so simple yet effective.



  • Always make sure that it works before you start

This is very important when talking about how to use a 12V circuit tester. Namely to avoid making a bad diagnosis and therefore a bad repair.

You should do this especially if you have an older tester that you don’t use much.

So, connect it to the battery before you start and see if the light bulb works. Sometimes the bulb will burn out or there may be a weak contact. If so, this may result in a bad diagnosis of the problem making you think something else is wrong when in fact the circuit tester may be faulty.

  • Make sure the clamp has a good connection

Always firmly connect the clamp to a clean connection, especially if you’re connecting it to the minus ground connection. Lots of mistakes and wrong diagnostics were made when the clamp was connected to parts that aren’t properly grounded. If needed, even use an extension cable to get a proper connection.

  • Avoid causing short circuits with the probe

The tip of the probe isn’t insulated. When checking connections, like light bulb holders, for instance, you may accidentally cause a short circuit. Although most installations are protected by fuses, it’s best to avoid this altogether.

As mentioned, if you’re going to use the probe to poke the cable, be careful not to damage it.



An alternative to a circuit tester is of course a multimeter. It’s an equally effective solution with much more options but more expensive and complicated.

We’ve shown this in a separate article about how to check alternator voltage output where you can see one way to use a multimeter.

Circuit testers are easy to find and buy, every hardware and car parts store surely has them for sale. If you have the chance, buy more quality ones, with a better cable and probe. The one in this example is several decades old but still works although it’s a bit beaten up.

Also, most of these can be maintained. Every part can be easily replaced (the bulb, cable, clamp, even probe) for an insignificant amount of money.

There is also the option of buying a more modern circuit tester with led lights inside.

Whatever kind you choose, buy and have one around the car. It may easily get you out of a tight spot and return the money in no time.


Written by: Sibin Spasojevic


Former car technician, life-long car and DIY enthusiast, author for