P0504Powertrain
Brake switch — signal mismatch
Where to start
The two brake pedal switches disagree. The pedal has two precisely so the unit can compare them. Usually the stop-light switch itself has shifted or worn out — a cheap part, replaced without removing anything. Symptoms: cruise control will not engage and the gearbox will not come out of Park.
A code is not a diagnosis. It says which check failed, not which part to replace. Clearing it without repair only means the car will report it again in a few trips.
How to put together a report a mechanic will actually read
We have a detailed piece on thisCruise control gone: codes P0500, P0501, P0504, P0571
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code: the same on every car
Which part of the engineIdle and auxiliary systems
Number of the faultThe serial number inside that group — it does not say how serious the fault is
All codes in «Idle and auxiliary systems»
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.