P0504: Brake Switch A/B Correlation
What to do
Driveable, but check the brake lights are working — if they are not, the car is unsafe in traffic and needs immediate attention. Cruise control will not engage and the car may refuse to shift out of park. Mention any recent work in the driver's footwell.
What this code means
The two halves of the brake pedal switch disagree. Brake switches are deliberately redundant — one circuit closes when the pedal is pressed and the other opens, so the module can tell a genuine brake application from a failed switch. When the two stop agreeing, it no longer can.
The consequences reach further than the brake lights. Cruise control will not engage without a trustworthy brake signal, many automatics will not shift out of park, and stop-start will not operate. A misadjusted switch after pedal or carpet work is a surprisingly common cause, and worth asking about before condemning the part.
Likely causes
Start here
- Faulty brake light switch · easy
- Corroded or loose connector · easy
Less common
- Broken wire in control circuit · moderate
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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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
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