P0504

Code

Generic P — Powertrain

Brake Switch A/B Correlation

Brand: Generic

Type: P — Powertrain

Views: UK: 37 EN: 111 RU: 66

AI status

Completed

ready

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Page language: EN

Causes

Faulty brake light / brake pedal position switch (one or both switches)
Misadjusted or sticking brake switch (mechanical alignment)
Damaged wiring or poor connector connection between switches and control module
Short to voltage or ground on one circuit causing inconsistent readings
Corroded grounds or poor chassis ground
Aftermarket electrical modifications (alarms, trailer wiring) affecting circuits

Symptoms

Malfunction Indicator Lamp (MIL) or ABS/Traction lights illuminated depending on network logic
Cruise control disabled or will not set/resume
Brake lights may be inoperative, stuck on, or intermittent
Shift interlock may not release (vehicle won’t shift out of Park) on some models
Reduced or disabled adaptive braking/driver assist functions
Possible no-start inhibit on some vehicles if brake input logic used for start/stop

What to check

Retrieve freeze frame and live data with a scan tool; monitor both Brake Switch A and B states while operating the pedal
Visual inspection of brake switch(s), mounting, and pedal linkage for damage or misadjustment
Inspect connectors for corrosion, bent pins, water intrusion and secure fit
Check for Diagnostic Trouble Codes in related modules (ABS, BCM) and for communication errors
Measure supply voltage and ground at switch connectors with key on, engine off
Continuity/resistance check of switch circuits and between switches and control module

Signal parameters

Brake Switch A/B states: expected to change together (e.g., both 0/1 or OFF/ON) within the same pedal travel
Typical logic voltage: LOW ≈ 0–0.5 V, HIGH ≈ 4.5–12 V depending on vehicle design (confirm with service manual)
Ignition-switched 12 V supply present at switch when required
Continuity: near 0 ohms when closed, open/infinite when open (verify per circuit design)
Scan tool values: switch status bits or percent indicators should match physical pedal position

Diagnostic algorithm

Connect a OBD-II scan tool and record freeze frame and live data for both Brake Switch A and B while cycling the brake pedal slowly and rapidly. Note any mismatches, delays or chatter.
Visually inspect switch mounting, plunger travel and pedal stop. Confirm both switches are actuated by the pedal at the same travel point.
With ignition ON, measure reference voltage and ground at each switch connector. Compare to vehicle spec. Check for voltage drop during operation.
Operate the brake pedal while probing signal wire and observe voltage transitions. Verify both circuits change state together. If one circuit is incorrect, isolate that circuit.
If a wire fault is suspected, disconnect switch and perform continuity/resistance checks back to the control module connector. Repair/replace damaged wires or connectors.
If switches and wiring check good, swap or bench-test the switch(es) (if removable) or replace the suspect switch and re-test.

Likely causes

Single faulty or intermittent brake switch
Switch plunger out of adjustment so A and B signals change at different pedal positions
Wiring harness damage at the pedal area or connector corrosion
Intermittent ground or power feed to one switch
Connector pushed out or pin damage at switch or module

Fault status

⚠️ Status

Brake Switch A/B Correlation — PCM detected inconsistent or non-correlating signals between dual brake switch inputs indicating a possible switch, wiring, or module fault.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours

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