Brake light switch out of adjustment, worn or faulty; Wiring or connector fault at the brake switch (corrosion, loose pins, chafed wires); Brake pedal stop or plunger worn or missing, so the switch does not move fully; Blown brake light fuse or a fault in the brake light circuit affecting the signal; Fault in the engine control unit or a related control module that reads the switch inputs; Incorrect aftermarket fitting or damage to the pedal assembly after previous repairs.

P0504

Brake Switch A/B Correlation.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
04: FaultThe specific fault in that area

What it means

P0504 means the engine control unit has compared the two brake switch signals (A and B) and found that they do not agree. The two signals should change state together when the brake pedal is pressed or released. If one reports 'pressed' while the other reports 'released' for longer than a set time, the code is stored.

Symptoms

Brake lights staying on or not coming on at all
Cruise control not working or cancelling unexpectedly
Engine idle speed higher or lower than normal
Reduced throttle response or limited power on some models
Difficulty starting on models that need the brake pressed to start
Warning light on the dashboard, possibly with a cruise control or brake light warning

Possible causes

Brake light switch out of adjustment, worn or faultylikely
Wiring or connector fault at the brake switch (corrosion, loose pins, chafed wires)likely
Brake pedal stop or plunger worn or missing, so the switch does not move fullypossible
Blown brake light fuse or a fault in the brake light circuit affecting the signalpossible
Fault in the engine control unit or a related control module that reads the switch inputsrare
Incorrect aftermarket fitting or damage to the pedal assembly after previous repairsrare

How to diagnose

Read the freeze frame data and any other stored codes, and note whether the brake lights have been behaving oddly
Check that the brake lights come on and go off properly when the pedal is pressed and released
Inspect the brake switch, its connector and the wiring for damage, corrosion, moisture or loose pins
Check the pedal plunger or stop and the switch position for wear and correct adjustment, as the layout varies by model
View the live data for both brake switch signals and confirm they change together as the pedal is pressed and released
Check the relevant fuses and the brake light bulbs for faults
Test the switch circuit for voltage, earth and continuity against the manufacturer's wiring diagram, as pin layouts depend on model and engine
If the switch and wiring are good, check the control unit's inputs and look for any applicable software updates for your model

How to fix

Adjust or replace the brake switch and make sure the pedal contacts it correctly
Repair or replace damaged wiring and clean or replace faulty connectors
Replace a worn pedal stop or plunger
Replace blown fuses and faulty bulbs after finding why they failed
Repair or replace the control unit only once all other causes are ruled out
Clear the code after the repair and road test to confirm it does not return

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but faulty brake lights are a safety and legal concern, so have it fixed soon. If the brake lights are not working properly, do not drive it until they are repaired.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.