Brake Switch A/B Correlation
What it means
P0504 means the two independent signals from the brake pedal switch (circuits A and B) disagree about whether the pedal is pressed. The car reads both to run brake lights, cruise control, the shift interlock, and, on some vehicles, push-button start. A misadjusted or failing switch, or a wiring/fuse fault, is the usual cause. The engine still runs, but the affected safety and convenience functions make this worth fixing quickly.
Can I keep driving? The car usually drives, but cruise control, brake lights, and on push-button cars even starting can be affected — get it fixed promptly.
Common symptoms
What you'll notice
- Cruise control won't engage or drops out
- Brake lights behaving erratically or staying on/off
- Check-engine light on
- Shift interlock or push-button start may not release (won't shift out of park or won't crank)
Common causes
Most likely first
- Misadjusted or faulty brake pedal switchLikely
- Wiring or connector fault in the brake switch circuitCommon
- Blown brake-light fuse affecting one of the two signalsCommon
- Faulty control module reading the switch (less common)Less often
How to diagnose & fix
Work through it in order
- Scan live data and compare the two brake switch signals as the pedal is pressed.
- Check the brake pedal switch adjustment and the striker/plunger at the pedal.
- Inspect the switch connector, wiring, and brake-light fuse.
- Replace the brake pedal switch if the two signals don't agree correctly.
- Verify both signals track together after the repair. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0504?+
Can I just clear P0504?+
Related codes
Often seen with P0504
Read & clear P0504 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.