Randomly flashing headlights after you’ve shut the car off may lead you to think your car is haunted. But rest assured, it’s not. At least not unless none of the causes we will discuss turn out to be true, then I would start toworry and consider alternative options.
Flashing headlights after you turn the car off can indicate a problem with the light control module, body control module, headlight relay, headlight switch, bad ground connection, or a faulty aftermarket alarm system. All of these are fairly easy to diagnose.
Understanding the Electrical System In Your Car
The lighting system in your car may seem pretty straightforward at first glance. And for the most part, it is. The main components are the bulbs, wiring, and the switch. The battery supplies power to the bulbs, which is carried through the wiring, and the whole process is engaged by the switch.
But there is also a relay, light control module or body control module, and possibly an alarm system. The relay is essentially a secondary switch for the lights. But keep in mind that not all cars have it. The same goes for the light or body control module. The two are only present in modern cars, but even so, not all of them have it.
The light control module is made so the lighting system can communicate with other systems in the car. Like the lights turning on after you unlock the car, after you open the doors, the automatic headlight function, and more. And most aftermarket alarm systems are connected to the horn, turn signals, and headlights. When the alarm goes off, the headlights flash in an effort to deter the burglar.
All these systems and components can be the possible cause for your headlights flashing. But the point is to do model-specific research before you start diagnosing the problem. Trying to find a light control module in a car that doesn’t have one obviously won’t get you far. The same goes for other components that are not universally present in all cars.
5 Common Causes of Flashing Headlights With Solutions!
Are your headlights flashing when your car is off? It could be a sign of a problem with your battery, alternator, or wiring. Learn how to identify the cause and try to fix the problem yourself.
1. Faulty Headlight Switch
Anyone with years of driving experience knows that headlight switches don’t fail, almost ever. They don’t have a maintenance schedule or an expiry date. But they can still be defective, and in some cars, a failing headlight switch is a common problem. In the mid-2000s, Mercedes models especially.
2000s Ford models are not immune to headlight switch failure either. And it fails just as frequently in Jeep models that have a headlight switch on the indicator stalk. If this turns out to be the problem, your only option is to replace the switch. A new switch will set you back between $40 and $80, or around $100 if it’s on the indicator stalk.
How To Diagnose
Diagnosing the headlight switch is fairly simple. You disconnect the switch and wait to see if the problem occurs again. The headlight switch only sends a signal to the relay or the control module to turn the headlights on. And if the headlights keep turning on after removing the switch, it means the problem is somewhere else.
2. Light Control Module or Body Control Module
Most modern cars have a light control module. But in some cases, the light control module is integrated into the body control module. It’s hard to say why a control module would fail because they are pretty bulletproof for the most part. But the two most common causes are water leaks and factory defects. If you suspect the light control module might be the cause, check for water damage first around its location.
If you find any, it’s more than likely a clogged drain pipe if it’s in the interior. Drain pipe layouts are different for every car, so that’s a whole subject on its own. It’s also worth mentioning that this is the most common cause of the problem at handfor BMW models. But it can happen with other manufacturers too.
Whatever the case, your only option is to replace the module. I suggest getting a used one because these rarely fail, and they are pretty expensive when new. A light control module for a Ford is over $250, and for a premium European brand, you can be sure it’s significantly more. Used control modules, even for a BMW, are less than $100.
How To Diagnose
The best way to diagnose a light or body control module is to scan the fault codes. Fault codes related to these two control modules are B0001 to B9999. If you notice multiple fault codes coming back, all seemingly unrelated to one another, there is a 99% chance it’s faulty.
And it would be even better to use a high-end scan tool that will tell you about communication issues between modules. That way, if a light or body control module fails to communicate with an X module, you can be 100% sure what to do.
3. Faulty Headlight Relay
Virtually all major electrical components in your car have relays. That makes the wiring simpler, cheaper, and more efficient. Instead of having thick cables going to each switch, you have smaller signal wires. Those signal wires control the relay, which acts as the main switch between the battery and, in this case, the headlights.
The problem with relays is that they are partially mechanical. The signal from the switch engages an electromagnet that pulls a thin metal tab that closes the electrical circuit. Those metal tabs wear out over time and start malfunctioning. Relays are pretty cheap and go for $10 to $30, plus they are just as easy to replace as a fuse. So, hopefully, this is the cause.
How To Diagnose
The headlight relay is usually in the engine bay fuse/relay box. You will have to check your owner’s manual to see which of the relays is for the headlights. You can also find the diagram under the relay box cover in most cars. Once you find the relay, remove it and remove the plastic cover from the top.
That will reveal the metal tabs inside. Check if the metal tabs look burnt, worm out, or have holes in them. If the answer is positive, replace the relay. But if you don’t see any signs of failure, check the relay box terminals for corrosion. And if that checks out as well, you can test the relay with a multimeter. Check the video below to see how.
4. Bad Ground Connection or Wiring
The ground connection is another very common cause of headlights flashing when the car is off. Only in this case, we are not talking about the headlight ground connection but the main ground strap. The ground strap goes off the negative battery terminal to the chassis and is the main grounding point for the entire electrical system.
When the ground strap is bad, your car may experience all kinds of electrical gizmos, but this headlight issue can be the sole symptom. In most cases, when the ground strap is bad, you can clean it, tighten it or replace it. A new ground strap is less than five bucks at any hardware store, so that’s good news. But if the ground strap is good, I’m afraid you will have to inspect the headlight wiring.
How To Diagnose
Locating the ground strap is pretty simple. Find your battery, and follow the negative lead to the chassis. At the chassis anchor point, check for signs of corrosion and if the bolt is loose. Then, clean the anchor point with contact cleaner spray and a wire brush, and tighten the strap down.
But if you notice insulation damage on the lead, it’s best to replace it. As for headlight wiring, you can do a quick visual inspection. And if you don’t find anything, it’s best to have a professional test the wiring because it’s not a simple task.
5. Faulty Aftermarket Alarm System
Right off the bat, I can tell you that aftermarket alarm systems are a nightmare to work with. Those splice into several different systems, including the ignition, immobilizer, lights, horn, and more. And the installation jobs are semi-professional at best in most cases.
And with such bad installation jobs, it’s not surprising that they can frequently malfunction in all kinds of ways, including headlight problems. The best course of action here is to remove the system altogether instead of trying to fix it. And even that will be extremely time-consuming, not to mention expensive.
How To Diagnose
The easiest way to diagnose the alarm system is to wait for the headlights to start flashing. If the headlight flashing seems controlled instead of random, chances are it’s the alarm system. In other words, if the headlights turn on and off for the exact same period each time, it means it’s controlled. With all other causes, the flashing will be erratic.
Understanding the Impact on Battery Life
If you drive a modern car with LED headlights, there is not much cause for concern over battery life. LED lights consume a ridiculously small amount of current, plus modern cars have very powerful charging and battery systems. Both of which lower the chances of flashing headlights depleting the battery. But that’s if you drive the car every day. If you leave it for a week or more, you will more than likely return to a dead battery.
But if you have an older car with halogen and especially xenon headlight bulbs, a lot of owners who experienced this problem reported the battery depleting overnight. Of course, that depends on how often the headlights flash, the battery age, and how big the battery is, but the point is it can happen.
As a reference point, when the headlights are constantly on without the engine running, they can deplete a healthy battery in about five hours. You can now consider your battery life and how often your headlights flash to figure out roughly how long it will take them to deplete your battery.
Conclusion
Flashing headlights are a peculiar problem not that many owners face. As a matter of fact, most drivers won’t encounter this issue ever. Luckily, it’s fairly easy to diagnose in most cases, and you should start with the headlight switch, control module, and ground strap. Those are the most common causes, especially if you drive one of the brands we mentioned. After that, move to other causes or have a professional inspect the car.
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