C0040: Brake Pedal Switch A
What to do
Check your brake lights are working — this switch usually operates them, and a car braking without brake lights risks being hit from behind. You may also find cruise control will not engage or the car will not shift out of Park. The part is inexpensive and often just needs adjusting, so this is one of the cheaper faults in this range.
What this code means
Brake pedal switch A has a circuit fault.
The brake pedal switch is one of the most safety-relevant cheap parts on a car, and it is usually doubled for exactly that reason.
Switch A and switch B are two contacts in one assembly, wired opposite ways round — one closes as the pedal moves, the other opens. A module that sees both change together believes the pedal; a module that sees only one change knows something is wrong without having to guess which.
That redundancy exists because so much hangs on the signal: brake lights, cruise control cancellation, the shift interlock that stops you leaving Park, brake override on a drive-by-wire throttle, and the ABS module's knowledge of whether you are braking at all.
It is a common failure and it usually announces itself elsewhere — cruise control that will not set, a car that will not come out of Park, brake lights that stay on or never come on. Adjustment is as common as failure: the switch is set against a striker on the pedal arm, and a pedal stop that has perished lets the switch sit wrong.
Likely causes
Start here
- Faulty brake light switch · easy
- Corroded or loose connector · specialist
- Damaged wiring harness · specialist
Rare
- Faulty stability control module · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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