Brake Switch A / B Correlation
Your car's brake switches aren't agreeing with each other about whether the brakes are being pressed. Think of it like two security guards at a door who keep giving different answers about whether someone entered.
The ECM monitors dual brake switch inputs (typically a primary and redundant switch) to verify brake pedal status for safety systems like cruise control and emission controls. The switches should activate and deactivate in correlation; if their signals diverge beyond acceptable timing, the fault triggers.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Brake Switch A to B Activation Delay | < 100 milliseconds correlation | > 100 ms or no correlation detected |
| Switch Signal State Agreement | Both switches match (on/off simultaneously) | Switches in disagreement for > 1 second |
Code P0504 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0504 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.