Brake lights that stay on, and the switch behind them

The symptom, and what it points at

If the brake lights, the cruise control and the shift interlock all stopped behaving in the same week, you are almost certainly looking at one switch rather than three faults.

One switch, several jobs

The brake light switch is a small plunger mounted on a bracket above the brake pedal arm. When the pedal is at rest it presses the plunger in. When you press the brake, the pedal moves away and the plunger extends, closing a circuit.

What surprises people is how much else depends on that one signal. The brake lights, obviously. But also the shift interlock, which will not let you move the lever out of Park until the car is satisfied your foot is on the brake. Also the cruise control, which uses the same signal to cancel the moment you touch the pedal. On many cars the ABS module, the stop-start system and the traction control all watch it too.

That is why a failed switch produces such a confusing spread of symptoms. Lights that behave oddly, a shifter that will not move, cruise control that stopped working, and sometimes a warning light on the dash, all at once, from one part that costs very little.

It also explains why chasing them individually is a waste of time. When several of those appear together, the common factor is the switch, and confirming that takes a few minutes rather than an afternoon.

The rubber stopper nobody knows about

There is a specific and very common failure worth knowing about, because it has a genuinely cheap fix.

Behind the brake pedal, where it rests against its bracket, most cars have a small rubber or plastic pad. Its job is to sit between the pedal and the switch plunger, holding the plunger in when the pedal is at rest. Over fifteen or twenty years that pad dries out, crumbles, and falls into the footwell as a few black fragments.

Once it is gone, the pedal no longer reaches far enough back to press the plunger in. The switch reads that as your foot on the brake, permanently. The brake lights come on and stay on, with the engine off and the key out of the car, until the battery is flat.

The tell is finding small pieces of hard black rubber on the carpet under the pedals, which people usually sweep up without a second thought. Look for a small hole in the pedal arm where the pad should be sitting.

It is worth a look with a flashlight before assuming anything more complicated, because that pad is a very small part and it explains one of the more expensive-looking symptoms on this page.

Why lights stuck on flattens the battery

Brake light bulbs draw real current. A pair of conventional filament bulbs pulls in the region of four amps, and a typical car battery has around fifty amp-hours of usable capacity when healthy.

Do the arithmetic and a car left with its brake lights on has roughly overnight before it will not start. It also has no warning: nothing on the dash tells you the brake lights are on, and from the driver's seat you cannot see them.

This is one of the most common causes of the parasitic drain that people spend money chasing. The car goes flat repeatedly, a new battery gets fitted, it goes flat again, and the switch was the answer the whole time.

The quick check is to walk behind the car with everything off and the key out. If the brake lights are lit, you have found it. If you have a garage, the giveaway is a red glow on the wall after you have locked up.

Worth mentioning as a temporary measure: pulling the brake light fuse stops the drain and gets you through a night, but it also disables the shift interlock and the third brake light. It is a way to save a battery while you wait for an appointment, not a way to drive around.

When the lights never come on at all

The opposite failure is less obvious to you and considerably more dangerous, because you have no way to notice it from inside the car. You find out from a ticket, or from someone stopping very close behind you.

Start with the simplest possibility. If one lamp is out and the others work, that is a bulb or a corroded socket, not a switch. A switch failure kills all of them at once, including the high-level third light.

If none of them work, check the fuse before anything else. Brake lights usually have their own circuit and a blown fuse takes the lot.

After that it is the switch, which fails open as readily as it fails closed, or the wiring and connector behind it. On cars that have had aftermarket alarms or remote starters fitted, the brake circuit is a common tap point and a badly done installation is worth suspecting.

There is one useful cross-check: if the brake lights are dead and the shifter also will not come out of Park, that combination confirms the switch rather than the bulbs, since the interlock takes its signal from the same place. Most cars have a manual interlock override near the shifter for exactly this situation, and the owner's manual will show you where.

What we do, and what it costs

We come to the car and test rather than replace hopefully. The switch gets checked for continuity in both pedal positions, the plunger for correct adjustment, the pedal stopper for the failure described above, and the circuit for voltage at the lamps.

The scan matters more than people expect on modern cars, because a switch fault frequently sets codes in the ABS or body control modules, and reading those confirms the diagnosis in seconds. It also catches the cars where the fault is in the module rather than the switch, which is less common but does happen.

Where it is the switch, it is a fast job and we carry the common ones. Where it turns out to be wiring, a connector or a poorly installed accessory, we trace it and show you what we found.

Diagnosis starts at $95 and covers the electrical testing and the scan. That is the diagnostic rate rather than our per-axle brake labor, because replacing a switch is not a brake job and pricing it as one would be dishonest. Any parts get quoted before we fit them.

We cover Flushing and the rest of Queens, Brooklyn, Manhattan, the Bronx, Staten Island and Long Island. If the car is flat because the lights have been on all night, tell us and we will bring a jump pack along with the tools.

Common questions