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Car or Truck Brake Lights Not Working How to Diagnose Tail Lights

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Created on: 2020-08-06

This video shows you how to diagnose a problem with your brake lights.

In this video, I'm going to show you what to check if your brake lights don't work and help you fix the problem. You may not even know that your brake lights aren't working unless someone tells you or you get pulled over for it. Some vehicles have a warning light on the dash that will actually give you an indication that they're not working, but for most of us, you're not going to know.

So, what you need to check first is what brake lights are out. If they're all out, or if only one is out, or just the bottoms are out, or just the third brake light is out. All right. We'll start with an easy one. Now, what if you just have this left side brake light out? The most likely cause is just a light bulb, but there is certain vehicles that have a dedicated fuse for the left side and the right side. It could be different. You could have a blown fuse. So, regardless of whether it's one bulb out or all of them, it's always a good idea to check your fuses.

Now, this vehicle has three different fuses for the brake lights. There is a stop lamp fuse right here under the hood. Grab that fuse, pull it out. Just take a look at it. Make sure it looks good. It's not burnt. It's not broken in the middle there. Or you could take a test light, and it's always a good idea to test all the fuses. You can take a test light and just go on the back side of the fuse. There is two little contacts that you can just touch, and as long as the light lights up on both sides, then you know that fuse is good. And when you're doing this, it's a good idea to have the key on so that all the fuses are powered up. And then there's two fuses in the inside of the vehicle. This stoplight vehicle stop, and then this one is actually for the high mounted third brake light. So, you needs to check those two. If this one was blown, just the two side bulbs would be out, would not be working, and then if this one was out, then the third brake light wouldn't work.

Now, if you had a blown fuse, you replace it, everything seems to be working, chances are it wasn't just the fuse. Fuses don't blow on their own. Could be something wrong with the wire. Could be something wrong with the socket, the connection of the bulb, or even a corrosion issue. Now, you could have some corrosion in the trailer wiring. If you notice that it was just the lower lights out, then that could be one of the causes, or even if you plugged in a trailer and there was something wrong with the trailer wiring, that could have taken out those lights and you didn't even know it. So, if the fuses are good, now we can push on the brake pedal and see if the lights work. Now, it'd be ideal if you had an assistant to help you push on the pedal, but if not, you can take a pry bar that fits and just move the seat in a position where it just pushes on the brake pedal. In this vehicle, you can actually have the brake lights work with the key out of the ignition. Some newer vehicles, you need the key in the on position, and, in that case, you probably want to have an assistant with you because if the vehicle came out of park, it's not going to be safe.

Now, if you have a bulb that looks like this, you can take the bulb, just take a look at the connections. See if you see any corrosion in there. It does look like this got hot a little bit, but there's nothing actually wrong with the socket. That's okay. That's normal. If you look at the filament inside and it looks obvious that it's broken or damaged, then obviously, you're going to need a bulb. But sometimes it may even look like this. It may look fine and it's still not making contact in the middle there. There's a little break in the wire that you can't see. Now, if you have another bulb that is the same type either from the other side, if the other side is working, you could do that, or even if you have another bulb to test, you can try that. See if that works. Or you could test the terminals with a test light. Just make sure you have power and ground. You just want to be careful not to touch any of the terminals together because they could arc out, but we have power there to test the ground. You would put this side of the test light on battery positive, and then go on the ground circuit and then just test the ground. And if the test light lights up, then your ground is good. Now, if you tested for power and ground and your test light lit up and you had both, then all you're going to need is a bulb. Sometimes it was easier to just start with the bulb if you had one on hand, but you don't always know. And for the third brake light, most vehicles you're going to have just two wires going to it, a power and a ground. The yellow wire is going to be the power wire, and black wire's going to be the ground. So, you can test it very similar to the other lights. To test these wires, you can actually take your test light, make sure someone is pushing on the brake pedal, and just do not force the test light in there. Just lightly touch it. And then I just put a pick on the other end. We can see if it lights up, and the light lights up. So, in that case, the third brake light would need to be replaced.

Now, if you're still trying to find one side working, one side not working, the next step would be to trace the wires to wherever they connect together before they get to the actual brake pedal. Now, sometimes it may go straight to the brake pedal. Sometimes it may be a junction block in between, so either the wiring is bad, the connection itself is bad, or something in the junction block, whether it's corrosion or the junction block itself, something is going to be bad. Now, if all the lights aren't working, the most likely cause is probably the brake light switch. Now, and you want to test the brake light switch, what you can do is take a jumper harness. Now, every vehicle is a little bit different. You want to disconnect the connector and find which wires actually go for the rear lights. On this vehicle, it happens to be the white and orange wire, so you can take that jumper harness and just go in between those two connections. And it's important to have a fuse there just in case you don't want to short anything out by accident. Jump those two wires, and then you can have an assistant check those lights and make sure they're working. And if those happen to be working when you're using the jumper harness, then you know the switch is bad. On newer vehicles, you can actually use a scan tool to do a lot of those tests. You can actually see when the brake pedal is applied on and off, and there may actually be a code saying that there's a problem with the brake pedal switch. In that case, it's a little bit easier to diagnose. One of the reasons why you're able to see all this data on the scan tool is the body control module is what's going to command the brake lights on and get the signal from the brake pedal itself. So, potentially, you could have a problem with the body control module, whether it be a program issue or even an internal fault.

So, I hope this video helped you out in understanding some of the problems that could go on with brake lights. If you enjoyed it, please subscribe to our channel. Make sure you ring the bell, turn on all notifications so you don't miss any of our videos.

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