Turn Signal Switch Failure: Why Your Brake Lights and Turn Signals Stop Working Together

When your brake lights and turn signals stop working at the same time, it's more than an inconvenience it's a safety risk that can get you pulled over or, worse, rear-ended. These two systems share common circuits on most vehicles, which means a single fault can knock out both. If you've noticed that pressing the brake pedal doesn't light up the rear and your turn signals are dead too, you're in the right place. This article walks you through the likely causes, how to narrow them down, and what to fix first so you can get back on the road safely.

Why would brake lights and turn signals stop working at the same time?

On most cars, trucks, and SUVs, the rear brake lights and turn signals share the same bulb filament (in dual-filament setups) or the same ground wire and connector. That shared wiring is efficient for manufacturers, but it means one broken link can disable both systems at once. Common shared points of failure include:

A bad turn signal switch the multifunction switch on the steering column routes power to both the brake lights and turn signal circuit. When it fails internally, both go dark.
A blown fuse some vehicles use one fuse to protect both circuits.
A corroded or broken ground wire the rear light assemblies rely on a solid ground connection. If it corrodes or snaps, neither brake lights nor turn signals will work.
A damaged wiring harness rodents, wear, or previous accident damage can sever the shared wiring between the fuse box and the rear of the vehicle.
Burned-out dual-filament bulbs in older vehicles with 1157-type bulbs, both filaments can fail at once.

Is it safe to drive if both brake lights and turn signals are out?

No. Driving without functioning brake lights and turn signals is illegal in every U.S. state and most countries. Other drivers behind you have no warning when you slow down or change lanes. You can be ticketed, and your vehicle may fail a state inspection. If you must move the car to a repair shop, drive during daylight, avoid highways, and use hand signals though this is only a temporary workaround.

How do I figure out which part is causing the problem?

Start with the easiest checks and work toward the more involved ones. You don't need expensive tools for most of these steps.

Check the fuses first

Open your fuse box (usually under the dash or in the engine bay) and look for the fuse labeled "STOP" or "TAIL" or "TURN." Your owner's manual will show the exact location. Pull the fuse and inspect the metal strip inside. If it's broken or blackened, replace it with one of the same amperage. If the new fuse blows right away, you have a short circuit somewhere in the wiring.

Inspect the bulbs and sockets

Remove the rear light housings and look at the bulbs. With dual-filament bulbs (like 1157 or 3157), check if the glass is darkened or if either filament is visibly broken. Also look inside the socket for green corrosion or melted plastic. Clean corroded contacts with electrical contact cleaner and a small wire brush. Replace any damaged sockets.

Test the ground connection

A weak or missing ground is one of the most overlooked causes. Find the ground wire attached to each rear light housing it's usually a black wire bolted to the car's body. Remove the bolt, sand off any rust or paint on the body, and reattach it tightly. A poor ground can make lights behave erratically or not work at all.

Test the turn signal switch

The multifunction switch (turn signal switch) on the steering column is a frequent culprit, especially on GM vehicles. This switch internally routes brake light power through its contacts. When those contacts wear out or break, both the brake lights and turn signals lose their connection. You can test the turn signal switch for brake light circuit continuity using a multimeter to confirm whether it's passing voltage through correctly.

Check the wiring harness

If the fuse, bulbs, ground, and switch all check out, inspect the wiring running from the front of the car to the rear. Look for chafed, pinched, or chewed wires, especially where the harness passes through the trunk, under the vehicle, or through rubber grommets. A wiring diagram helps here you can follow a wiring diagram to trace the circuit from the fuse to the rear lights and identify the break.

What if the third brake light works but the tail brake lights and turn signals don't?

This is actually a helpful clue. The third brake light (center high-mount stop lamp) often runs on a separate wire from the turn signal switch. If it lights up when you press the brake pedal but the rear brake lights and turn signals don't, the problem is almost certainly inside the turn signal switch because that switch controls the path to the lower brake lights specifically. You can diagnose this specific scenario step by step to confirm the switch is the failure point.

Which vehicles are most likely to have this problem?

While any vehicle can experience this, some are known for it:

GM trucks and SUVs (Silverado, Sierra, Tahoe, Suburban) the multifunction switch is a well-documented weak point on many model years from the late 1990s through the 2010s.
Ford F-150 and Expedition wiring harness corrosion near the rear connectors is common in northern climates where road salt is used.
Dodge/RAM trucks similar multifunction switch issues, especially on 2002–2008 models.
Older vehicles with 1157 bulbs dual-filament bulbs burning out simultaneously (or one filament shorting to the other) can cause both systems to fail.

What are the common mistakes people make when troubleshooting this?

Replacing bulbs without checking the fuse first. The fuse is a 30-second check. Always start there.
Ignoring the ground wire. Many people spend hours replacing parts before checking whether the rear housing has a solid ground.
Assuming the problem is at the rear of the car. Since both brake lights and turn signals are affected, the fault is often upstream at the fuse, the switch, or the wiring between them.
Not using a wiring diagram. Guessing at wire colors and connector locations wastes time. A diagram specific to your vehicle's make, model, and year keeps you focused.
Skipping the multimeter. Visual inspection catches obvious damage, but a multimeter tells you whether voltage is actually reaching each point in the circuit.

How much does it cost to fix brake lights and turn signals that stopped working together?

It depends on the root cause:

Fuse replacement: Under $5 you can do it yourself in minutes.
Bulb replacement: $5–$20 per bulb, depending on the type.
Ground wire repair: Free to $10 if you clean and reattach the existing wire.
Turn signal switch replacement: $30–$150 for the part (aftermarket), plus $100–$200 labor at a shop. DIY is possible on most vehicles with basic hand tools and a steering wheel puller.
Wiring harness repair: $20–$50 in supplies (wire, connectors, heat-shrink tubing) if you do it yourself; $100–$300 at a shop depending on the extent of damage.

What should I do right now if both systems are dead?

Check the fuse. Pull it, inspect it, replace it if blown.
Look at the bulbs and sockets. Remove the rear housings and inspect both filaments and contact points.
Check the ground wires. Sand and retighten the ground connections at each rear light.
Test the turn signal switch. Use a multimeter to check continuity on the brake light circuit through the switch.
If the third brake light works but the others don't, focus on the turn signal switch it's the most likely cause at that point.
Inspect the wiring harness if everything else checks out.

For more in-depth electrical theory and wiring diagrams, WiringProducts.com has useful reference material on automotive electrical systems.

Quick checklist before you call a mechanic

✅ Fuse inspected and replaced if needed
✅ Bulbs checked for broken filaments or darkened glass
✅ Sockets inspected for corrosion or melting
✅ Ground wires cleaned, sanded, and retightened
✅ Turn signal switch tested with a multimeter
✅ Wiring harness visually inspected from front to rear
✅ Third brake light tested separately to narrow down the cause

If you've worked through all these steps and still can't find the problem, a shop with a scan tool and wiring diagram access can trace the fault faster. But in many cases, the issue turns out to be the turn signal switch, a bad ground, or a simple blown fuse all of which you can handle at home with patience and a multimeter.

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