When one or more of the car headlights stops working, a lot can go wrong because of limited visibility. However, these lighting fixtures can also fail in the completely opposite direction. This is when one headlight stays on when the car is off.
Whenever this happens, the most common culprits are a faulty engine timer, faulty headlight switch, bad headlight relay, or a faulty light sensor.
This can also be caused by water pooling in the headlights. The following is a detailed troubleshooting guide for a headlight that won’t turn off.
Why is one of my headlights not turning off?
Figuring out why one of your headlights remains on after shutting down your vehicle is a little bit tricky.
That’s because there’s a wide range of headlight systems, each of which works differently. To pinpoint the underlying issue, use our step-by-step diagnosis list:
A step-by-step way to diagnose a faulty headlight
Here’s the step-by-step list:
- Start the engine and ensure it is running.
- Check the lights, including the main beam, dip, and side lights, to ensure they are working properly.
- Verify that the tail lights are functioning correctly.
- With the lights turned on, activate the indicator switch to the left for a few seconds, then repeat the process by turning it to the right. Observe any unusual behavior or abnormalities with the lights during this test.
- Unusual behavior with the lights while using the indicator switch can indicate an earthing fault (grounding) issue.
- Turn off the engine and listen for the lights-on warning alarm. If the alarm does not sound, there may be an issue with the electrical system.
- Examine the connectors for signs of corrosion, as corrosion can often cause light faults. Check all connectors thoroughly, ensuring they are secure and clean.
- If the connectors appear to be in good condition, proceed to check the relays. Faulty relays can also cause lighting issues.
- Another potential cause of the problem could be the switch itself. Gently wiggle the switch, feeling for any loose movement or flickering lights. If the switch is faulty, it may need to be replaced entirely.
- After completing the above steps, if the issue persists, it is recommended to inspect the wiring loom for any potential faults. However, wiring issues are relatively rare compared to other possible causes.
- Finally, if none of the previous steps have resolved the problem, test the bulbs using a circuit tester. Although rare, there is a possibility that a faulty bulb could be the cause of the lighting issue.
Remember, these steps are a starting point and may not cover every possible scenario.
If you are unsure or unable to troubleshoot the problem yourself, it is recommended to consult a professional mechanic or electrician.
Top 6 reasons one headlight stays on after your car turns off?
1. Bad timer in the engine
The headlight technology in cars has come a long way. In newer vehicles, drivers have the option to operate headlights either manually or automatically.
The automatic system works with a timer in the engine that signals the headlights to turn off 3 to 5 minutes after the engine has been turned off. If this timer fails, the headlights will stay on until it’s fixed.
Fortunately, this problem is easy to fix. Try applying the parking brakes and you’ll notice the faulty headlight go off.
2. Faulty relay
The relay is the part that connects the headlight to the car’s battery or electrical system. It uses a simple on/off mechanism to operate.
Each time you switch on the headlights, the relay also turns on; hence, activating the electrical connection that powers the lights.
If the relay malfunctions while the headlights are switched on, the lights remain illuminated until it gets fixed.
The question is, how can you establish that a faulty relay is to blame for the headlight? Well, start by locating the relay, which is typically housed in the fuse box.
Gently pull it out and observe what happens to the headlight. If it goes out, it means the relay is to blame.
But just to be sure, look for an alternative relay in the same fuse box and use it in place of the headlight relay.
If the lights work properly with the new relay, it means you have a faulty headlight relay.
3. Bad headlight switch
A bad headlight switch is more commonly linked to inconsistencies in the lights’ performance. For instance, some headlights will only work in high beams and then malfunction in low beams.
However, there are a few exceptions where a faulty switch can cause the headlights to stay on. In fact, if the relay isn’t the culprit, then a bad headlight switch is the second-most common issue.
Unlike the relay, diagnosing a bad headlight switch is a little more complicated. As such, this problem is best left to the professionals.
4. Daytime Running Light Module (DRLM)
The daytime running light module is a tiny electronic gadget that regulates the headlights’ low-beam setting.
It collects information from a variety of sensors and switches to determine the right time to activate this setting.
More specifically, it gathers input from the parking brake switch, ignition switch, ambient light sensor, headlamp switch, and the dashboard’s indicator lights.
A malfunction in this module can cause these daytime running lamps to stay on even after the car has been switched off.
Unfortunately, fixing the DRLM isn’t that easy. You’d need specific tools, such as a test light and multimeter.
Even if you have these tools, you should know how to read a wiring diagram. If you don’t have much technical expertise, consider hiring a professional mechanic.
5. Light sensor
For cars that use manual headlights, the driver has to turn them on/off manually.
However, other vehicles use automatic headlights. This latter system involves a light sensor positioned in the front part of your car.
This sensor detects changes in ambient light, causing the headlights to illuminate once the sun sets.
A faulty sensor means the headlights stay on throughout.
To check if this is the issue, try switching the auto system to “off” or “0”. If the headlight goes out, then you likely have a faulty sensor.
6. Water filling the headlights
Whenever you switch on your vehicle’s headlights, they generate both light and heat. As soon as you turn them off, the air inside the headlamp cools at a slower rate than the air outside causing condensation.
This process is what causes the headlights to start filling with water and it can lead to them staying on throughout.
Water can also get inside the headlights if they’re broken or have worn-out seals. If this is the case, it should be pretty easy to fix by replacing the worn-out seal with a new one or repairing the crack.
However, if the problem is caused by too much condensation, you may have to replace the entire headlight with a new one.
Quick Fixes to Prevent Battery Drainage
If you have a headlight that’s refused to turn off, you’ll want to get this issue fixed immediately. That’s because it can drain the battery, causing the car to either stall or refuse to start altogether.
If you can’t fix the faulty headlight(s) immediately, there are a few things you can do to prevent battery drainage, namely:
Detach the battery
If you don’t want the battery to drain, all you have to do is to disengage it.
Using a wrench or socket, disconnect one of the cables that connect the battery to the vehicle.
If you’ve never dealt with a car battery before, I’d recommend disconnecting the negative cable. Disconnecting the positive cable can easily lead to a short circuit.
For clarity purposes, the negative cable is the black one and it connects to a – symbol on the battery. Conversely, the positive cable connects to a + symbol and is usually red in color.
Once you’ve disconnected the cable, position it in such a way that it’s not close to the battery. This is to eliminate the risk of the cord coming back into contact with the negative battery terminal.
As soon as you disconnect the cable, the headlight will switch off. So you won’t have to worry about any battery drainage.
Disconnect the fuse
A simpler way to avoid draining the battery is to disconnect the headlight fuse. This part can be found in one of two places: underneath the dashboard or inside a fuse box located under the hood.
Most fuse box covers have a chart showing which fuse links to each element of the electrical system. So it shouldn’t be too hard to find the headlight fuse.
Disconnect the relay
Removing the headlight relay can also prevent the battery from draining. The relay is the big, rectangular box located right next to fuses inside a fuse box.
It’s designed to handle a large amount of electrical load. So removing it will certainly break the circuit between the battery and headlights.
Can a Bad Fuse Make One Headlight Go Out?
Yes, it can. A damaged fuse can also prevent the headlight from shutting off, especially when there’s a system overload or short circuit.
Oftentimes, such issues are caused by worn-out or faulty wires. If this is the case, all you need to do is to inspect and replace the broken fuse.
Conclusion
There are several reasons why one headlight stays on when the car is off. It could be that the relay, headlight switch, light sensor, or timer is faulty.
This can also be caused by water pooling in the headlights or a malfunction of the daytime running lights.
Whatever the cause is, it’s good to address this issue instantly to avoid battery drainage. Alternatively, consider disengaging the battery, relay, or fuse.
This will automatically stop the battery from powering the headlight.
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