The basic technology behind headlights hasn’t changed much in decades, and even newer systems like adaptive headlights aren’t flashy enough to get much attention. But what happens if you can’t get your headlights to turn off?
What to Do When Your Car’s Headlights Won’t Turn Off
When your headlights suddenly stop working, things can get dangerous in a hurry. But headlights can also fail in the other direction. Far from a fail-safe, headlights that won’t turn off, no matter what you do, can quickly drain your battery and leave you stranded.
With that in mind, the short-term solution for headlights that won’t turn off is to take emergency preventative measures to keep the battery from going dead. This can be accomplished in several ways:
- Disconnect the battery.Remove the headlight fuse.Remove the headlight relay.
Your headlights should now be disconnected. Even though headlight systems usually aren’t complicated, there are situations where you may have to take your car to a professional to fix headlights that won’t turn off. But before you do that, there are a handful of things you can do, using some basic diagnostic tools for cars, to fix the problem.
Possible Causes for Headlight Issues
Some of the issues that can cause car headlights not to turn off include problems with the following components:
- Headlight switchDaytime running light moduleLight sensorRelayGrounded wire
Properly diagnosing a headlight problem can be complicated because there are many types of headlight systems out there. For instance, some cars are designed so that when the engine is shut off while the headlights are on, they remain on for a specific amount of time. In that case, you might want to shut off the headlights before turning off the engine to see if that helps.
Other cars have daytime running lights, which automatically turn the headlights on—but don’t affect the dash lights—during the day. If that system fails, it may cause the headlights to remain on. You can set the parking brake to see if that shuts off the headlights, as setting the parking brake typically disables daytime running lights. Removing or replacing the daytime running light module will probably fix that problem.
Quick Fix to Prevent Your Headlights From Draining the Battery
If you don’t have time to tackle the problem right away, or you want to leave the car for a while without the battery dying, there are two ways to keep the headlights from draining the battery.
Disconnect the Battery
The easiest way to keep your car battery from dying is to disconnect it. This involves literally disconnecting one of the battery cables from the battery, which requires an appropriately sized wrench or socket.
If you’ve never disconnected a battery before, it’s a good idea to disconnect the negative cable instead of the positive cable to avoid causing a short circuit.
The negative cable is typically black, while the positive cable is typically red. You can also look on the battery for a - symbol, which will be near the negative terminal, and a + symbol, which will be near the positive terminal.
After disconnecting the negative battery cable, move it away from the battery so that it doesn’t get nudged or bumped and come back into contact with the negative battery terminal.
Once the battery is disconnected, the headlights will turn off, and the battery won’t drain.
Remove the Fuse or Relay to Turn Off the Headlights
The other way to shut off the headlights is to remove the appropriate fuse or relay. This is a little more complicated than disconnecting the battery because you must locate the correct fuse panel and then figure out which fuse or relay to pull. This will prevent a loss of power to the computer and radio, though, so you won’t have to deal with any fallout later on.
Disconnecting the battery may erase the onboard computer’s memory, so it will have to go through a “relearning” process to correct the fuel economy. If your car stereo has a security feature that requires a special code after a loss of power, find your car radio code before you disconnect the battery.
Bad Relays
If a bad headlight relay is a reason your headlights won’t turn off, the fix is to replace the relay. This is a little easier to check since there’s a chance that multiple circuits may use the exact same type of relay.
If you can find another relay in your car that has the same part number as the headlight relay, you can remove your headlight relay, swap it for the identical one from a different circuit, and see if the headlights turn off normally. If the headlights turn off, you need to buy and install a new relay.
In the event that swapping relays doesn’t work, the problem may be a bad headlight switch, multifunction switch, or light sensor, and the diagnostic procedure will be more complex. You may be able to identify the problem by removing the component in question and checking for physical damage, but there won’t always be physical indicators.
For instance, a bad headlight switch that’s shorted internally may get hot enough to crack, melt, or burn the plastic housing or electrical connections, but that isn’t always the case.
If you aren’t able to identify the malfunctioning component, disable the headlights by disconnecting the battery or removing the appropriate fuse, waiting for daylight, and then taking your car to a trusted mechanic.
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