Without effective brake lights, you can quickly find yourself in a rear-end collision, and a possible law enforcement prosecution. So you are right, this problem is a priority and luckily, I think you can fix this problem right now.
Brake lights on when headlights on? The problem with brake lights that come on with headlights is probably an incorrect bulb fitted. Other common reasons include:
- Bulb incorrectly fitted
- Faulty bulb
- Faulty tail light bulb holder circuit board
In this post we’ll look at all the likely causes in a little more detail and what you can do to fix them right now and probably without any tools.
Incorrect Bulb Fitted
Taillight bulbs come in two main categories, single filament or double filament.
A single filament means the bulb has only one circuit, and a double filament means it has two circuits, simple enough, right?
Each circuit controls a light, so the double filament bulb has two separate lights inside one bulb. One light within the bulb is reserved for the tail light and the other – the stoplight.
The double bulb saves space and money by squeezing two lamps into one.
The double filament bulb is also known as a “Double”, “Stop & tail” or “Twin”. And although it looks identical to a single filament bulb (aka single), it’s easily identified by its two contact points on the base of the bulb.
It’s also possible a higher output single stop bulb has been incorrectly fitted in the tail light holder. See the codes below.
The bulb’s code will be stamped into the metal base. Popular stop and tail bulb codes include:
- Popular single stop bulb code: P21w
Popular single tail light bulbs:
Fitting the wrong bulb is an easy mistake to make, the bulbs are the same dimensions, and so it’s possible to get a single bulb into a twin bulb holder. I’ve done it loads of times…..Schhh.
If you’ve recently done some work on the rear lights or had it serviced or maybe you just bought it, whichever, the wrong bulb fitted is most likely your problem.
Mistakenly fitting a single bulb to a twin holder connects the tail light circuit to the stoplight circuit. So when you turn on the headlights, you have brake lights too. Repairing this error is a simple fix.
To check if this is what has happened, go ahead and:
- Remove the bulb holder from the rear of both tail lights. This usually doesn’t require tools, just squeezing some plastic tabs.
- Remove the light fitting and remove the stop light bulb, push in and twist anti-clockwise.
- Check and verify if the holder is a twin or a single connector, (see pic above) the contact points in the holder will be evident.
- Match the correct bulb with the holder.
Bulb Fitted Incorrectly
The twin bulbs have a bayonet-style fitting, push-in, and twist to fit/remove. Pins at the base of the bulb connect with the bulb holder. The pins are offset to prevent incorrect bulb fitting.
Many of the off-set bulbs will fit in the tail light bulb holder backways with a little motivation. And when that happens, you’ve connected the light circuit to the stoplight bulb.
Now, turning on the headlights also turns on the stoplights.
To check if this is what has happened, go ahead and:
- Remove the tail light bulb holder.
- Remove the tail light bulb and pay attention to how difficult it is to remove.
- When removed, check that bayonet pins align with the channels in the bulb holder.
- Refit the bulb aligning the pins correctly, check both tail light holders.
The twin bulb is a marvel, no doubt, but it’s not without fault. It is possible for one of the bulb’s internal filaments to touch off each other, leaking power from the headlight circuit into the brake light circuit.
To test for this condition replace both the left and right stop & tail bulbs.
Faulty Tail Light Circuit Board
Faulty bulb holder circuit boards are common. They live in the trunk and often get knocked around. The metal circuits can get bent out of shape and contact other circuits, powering them up.
Taillight bulb holder circuit boards are usually very basic and even the least electronically minded DIY’er will spot an issue in a heartbeat.
So go ahead and identify the brake light circuit and also the light circuit. You can do this by simply following the stoplight holder metal circuits from holder to wiring connector.
Is there any sign of damage or shorting from one circuit to the other? If so, you’ll likely be able to repair it. If you’ve found no issues, go ahead and check the wiring connector and trunk loom for shorting.
Ask yourself questions like, have any accessories have been fitted to the car recently, make sure no fixing screws have damaged wiring. Does this problem coincide with any other events?
Are brake lights and tail lights the same bulb? Yes, most cars will have one bulb, known as a stop and tail bulb, that serves as a tail light and brake light. It is however two separate filaments within a single bulb.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.