Why are my brake lights stuck on?

The most common reason by a distance is a crumbled pedal stop pad. Behind the brake pedal there is a small plastic or rubber block that the pedal rests against when released, and it is what physically holds the brake light switch in its off position. These pads become brittle with age and eventually disintegrate, dropping into the footwell as a small black disc. With nothing pressing the switch, it thinks the pedal is down permanently and the brake lights stay on. The part costs very little and fitting it is a two-minute job once you know.

Will a faulty brake light switch fail an MOT?

Yes, and potentially twice from one part. The tester checks the stop lamps directly, so brake lights that do not operate, or that remain on when the pedal is released, are defects in their own right. Separately, if the switch fault has lit the ABS or stability lamp, a brake system warning device indicating a malfunction is a major defect with 85,196 recorded failures against it. It is an unusually cheap component to fail an MOT on.

Why does my automatic gearbox not come out of park with C0161?

Because of the shift interlock. Automatic cars will not let you move the selector out of park unless the brake pedal is pressed, and the car knows the pedal is pressed from the brake switch. With a faulty switch the interlock never releases, so the lever will not move. Most cars have a manual override — usually a small slot beside the selector that you press with a key or screwdriver — described in the handbook, which will get you moving, but it is a workaround rather than a fix.

Can I adjust a brake light switch myself?

Sometimes, but it needs care because getting it wrong can leave the brakes lightly applied. Many modern switches are self-adjusting: they are fitted in an extended position and set themselves the first time the pedal is pulled up or pressed, and if that sequence is done in the wrong order the switch ends up out of adjustment permanently. Older screw-in types can be adjusted by turning them in or out. In both cases, check afterwards that the lights come on with light pedal pressure and go out completely when the pedal is released.

Chassis code · standard

C0161Brake pedal switch signal not making sense to the ABS system

Standard definition: ABS/TCS Brake Switch Circuit

Last updated 27 July 2026

Quick answer

The ABS and traction control system is getting an implausible signal from the brake pedal switch. It is usually a cheap switch behind the pedal at fault, and the same switch normally operates your brake lights, which makes this an MOT problem in its own right.

How the car works this out

The brake pedal switch on nearly every modern car has two sets of contacts arranged to work in opposition: one closes as the pedal is pressed, the other opens. The ABS module reads both and expects them to disagree in a specific way at all times. If they ever agree — both saying pressed, or both saying released — one of them must be wrong. C0161 is set when that cross-check fails, or when the module sees a brake signal that contradicts what the wheel speeds and the accelerator are doing. That deliberate redundancy is why this fault is caught rather than silently accepted.

What you might notice

ABS, traction control or stability lamps on
Brake lights permanently on, even with the car parked and locked
Brake lights not coming on at all
Cruise control refusing to engage, or dropping out immediately
Automatic gear selector refusing to come out of park
Stop-start not working, or the engine restarting unexpectedly
A flat battery overnight where the brake lights are staying on

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A failed or maladjusted brake pedal switchCommon
How to check: Have someone watch the brake lights while you press the pedal very slightly. Lights that come on before the pedal has really moved, or not until it is well down, indicate an adjustment problem.
2A crumbled or missing pedal stop pad — the small plastic or rubber block the pedal rests againstCommon
How to check: Look at the bracket behind the brake pedal for a small plastic plug. If it has disintegrated and fallen into the footwell, the switch is never released and the brake lights stay on permanently. It is a very cheap part and a classic cause.
3A corroded or loose connector at the switchPossible
4Damaged wiring in the footwell loom, sometimes from feet or from aftermarket wiring run under the dashboardPossible
5A switch fitted incorrectly at a previous repair, so its self-adjusting mechanism was never setPossible
How to check: Many of these switches set their own position the first time the pedal is pressed after fitting, and getting that sequence wrong leaves them permanently out of adjustment.
6A fault in the ABS module's input circuitLess common

What to do about it

1Get someone to check your brake lights today. This code frequently comes with brake lights that either stay on all the time or do not work at all, and both are safety problems and MOT defects.
2Look behind the brake pedal for the small stop pad. If it has crumbled away, that is very likely the whole fault, and the part costs almost nothing.
3Have the switch checked and adjusted before replacing anything else. It is one of the cheapest components involved.
4Make sure any new switch is set correctly. Many self-adjust on first press and will not work properly if the sequence is not followed.
5Deal with permanently illuminated brake lights promptly. Besides being illegal and confusing to drivers behind, they will flatten the battery over a few days.

Can you keep driving?

Only to a garage

This one has a road safety dimension that most chassis codes do not. If your brake lights are not working, drivers behind you have no warning that you are slowing, which is a genuine hazard rather than a theoretical one. If they are stuck on, you are giving a permanent false signal and flattening the battery. Check which of the two you have before driving anywhere, and go straight to a garage.

Will it fail an MOT?

Two separate MOT issues, and the less obvious one is the more serious. The lamp is a major defect: a brake system warning device indicating a malfunction, with 85,196 recorded failures in the DVSA data behind this site. But the tester also checks the stop lamps directly, operating the pedal and confirming both lamps work, are the right colour, and are not affected by other circuits. Stop lamps that do not work, or that stay on permanently, are defects in their own right, and this is one of the few fault codes that reliably produces both problems from a single cheap part.

The light this turns on

ABS warning lightThe anti-lock braking system has found a fault and switched itself off. Your brakes still work exactly as they always did in normal driving — what you have lost is the system that stops the wheels locking and the steering going light under a hard, emergency stop.

Common questions

Why are my brake lights stuck on?: The most common reason by a distance is a crumbled pedal stop pad. Behind the brake pedal there is a small plastic or rubber block that the pedal rests against when released, and it is what physically holds the brake light switch in its off position. These pads become brittle with age and eventually disintegrate, dropping into the footwell as a small black disc. With nothing pressing the switch, it thinks the pedal is down permanently and the brake lights stay on. The part costs very little and fitting it is a two-minute job once you know.
Will a faulty brake light switch fail an MOT?: Yes, and potentially twice from one part. The tester checks the stop lamps directly, so brake lights that do not operate, or that remain on when the pedal is released, are defects in their own right. Separately, if the switch fault has lit the ABS or stability lamp, a brake system warning device indicating a malfunction is a major defect with 85,196 recorded failures against it. It is an unusually cheap component to fail an MOT on.
Why does my automatic gearbox not come out of park with C0161?: Because of the shift interlock. Automatic cars will not let you move the selector out of park unless the brake pedal is pressed, and the car knows the pedal is pressed from the brake switch. With a faulty switch the interlock never releases, so the lever will not move. Most cars have a manual override — usually a small slot beside the selector that you press with a key or screwdriver — described in the handbook, which will get you moving, but it is a workaround rather than a fix.
Can I adjust a brake light switch myself?: Sometimes, but it needs care because getting it wrong can leave the brakes lightly applied. Many modern switches are self-adjusting: they are fitted in an extended position and set themselves the first time the pedal is pulled up or pressed, and if that sequence is done in the wrong order the switch ends up out of adjustment permanently. Older screw-in types can be adjusted by turning them in or out. In both cases, check afterwards that the lights come on with light pedal pressure and go out completely when the pedal is released.

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