P0504
Brake Switch "A"/"B" Correlation
- Severity
- Medium
- Driving
- Drivable — book it in
The two brake switch signals disagree. Vehicles use two independent brake switch circuits precisely so that a single failure can be detected, and this code is that detection working: one of the two is wrong, and the module cannot tell which.
Possible causes
- CommonMisadjusted or failed brake pedal switch
- CommonWorn or broken striker on the pedal bracket
- CommonWiring or connector fault in the footwell
- PossibleBlown brake light fuse
- PossibleAftermarket alarm or cruise control spliced into the circuit
- CommonOne of the two switch circuits failed while the other still works
What you might notice
- Brake lights stuck on or not working
- Cannot shift out of park
- Cruise control will not engage or will not cancel
- Stop-start system disabled
- Check engine light on
What to check, in order
- Watch both brake switch states together on the live data screen while pressing the pedal. The one that does not change with the other is the fault.
- Check the switch adjustment - a switch the pedal only just reaches will change state later than its partner and set this code without either being broken.
- Look at the brake lights. Working brake lights that go off when the pedal is released eliminate most of this without any tools.
- Check the switch adjustment at the pedal - these are adjustable and drift as the plastic striker wears.
- Watch the brake switch state on the live data screen while someone presses the pedal.
- Inspect the connector in the footwell, where feet catch it.
- Ask whether an aftermarket alarm or cruise control was fitted.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Vehicle speed control, idle control and auxiliary inputs
P0500P0501P0502P0503P0504P0505P0506P0507P0508P0509P0510P0511P0512P0513P0514P0515P0516P0517P0518P0519P0520P0521P0522P0523P0524P0525P0526P0527P0528P0529P0530P0531P0532P0533P0534P0535P0536P0537P0538P0539P0540P0541P0542P0543P0544P0545P0546P0547P0548P0549P0550P0551P0552P0553P0554P0555P0556P0557P0558P0559P0560P0561P0562P0563P0564P0565P0566P0567P0568P0569P0570P0571P0572P0573P0574P0575P0576P0577P0578P0579P0580P0581P0582P0583P0584P0585P0586P0587P0588P0589P0590P0591P0592P0593P0594P0595P0596P0597P0598P0599