Powertrain, generic · Vehicle speed, idle control and auxiliary inputs
P0504 Brake Switch A/B Correlation
Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible
The brake pedal switch has two contacts – A usually for the brake lights, B for cruise control and brake information – working in opposite sense, so the module expects opposite states on every brake application. P0504 means they disagree. The signal is used for cruise control cut-off, brake override with electronic throttle, the shift interlock and start-stop. Ford sets it only after more than 25 (petrol) or 40 (diesel) disagreeing brake applications, with the wrench lamp instead of the warning lamp; a NHTSA investigation covers Hyundai brake light switches.
Likely causes
Brake light switch faulty (burnt or corroded contact)
Switch misadjusted or retaining clip or stop pad worn
Wire, connector or brake light fuse faulty
Moisture in fuse or junction boxes
Symptoms
- Cruise control cannot be switched on
- Brake lights stay on or do not come on
- Selector lever cannot be moved out of P
- Wrench or ESC lamp on
Repair estimate
DifficultyEasyDIY possible
Parts and labour€20–150
Time0.2–1 h
Typical range across vehicles; the real cost depends on what the testing finds.
Parts involved
brake pedal switchbrake light circuitengine control module
Related codes
References: fordservicecontent.com, fordservicecontent.com, static.nhtsa.gov, roushcleantech.com.