P0504: Brake Switch A/B Correlation
What Does P0504 Mean?
P0504 is a diagnostic trouble code indicating: Brake Switch A/B Correlation. This code relates to the vehicle speed and idle control system. It has been reported in NHTSA complaints across 2 different vehicle models.
The vehicle speed and idle control systems manage engine operation at rest and in motion. The Vehicle Speed Sensor (VSS) provides real-time speed data used for fuel calculations, shift timing, cruise control, ABS, and speedometer display. The Idle Air Control (IAC) system regulates airflow to maintain target idle speed (typically 600-900 RPM). On electronic throttle control vehicles, the ECM uses the throttle body motor directly. When the ECM detects faults in these systems, drivability problems like stalling, surging, incorrect speedometer readings, and cruise control failure are common. Speed sensor failures are often caused by metal debris from internal transmission wear accumulating on the magnetic sensor tip. Idle problems are most commonly caused by carbon buildup in the throttle body restricting airflow.
Symptoms of P0504
- ⚠️ Check engine light on
- ⚠️ Erratic or inoperative speedometer
- ⚠️ Unstable idle — surging, hunting, or stalling
- ⚠️ Cruise control not working
- ⚠️ Transmission shift quality issues
- ⚠️ ABS or traction control warning light
Common Causes
- 🔍 Faulty vehicle speed sensor or idle control valve
- 🔍 Carbon buildup in throttle body
- 🔍 Vacuum leak affecting idle control
- 🔍 Wiring issue in sensor or actuator circuit
- 🔍 ECM or instrument cluster fault
P0504 Reports by Year
Real Owner Reports
From NHTSA complaint database — actual owner descriptions.
"THIS IS A KNOWN ISSUE ON THE 2015 MUSTANGS. THE RECALL ONLY INVLOLVES VEHICLES FROM CERTAIN STATES WITH HIGH TEMPS AND OR HUMIDITY BUT DOES NOT INCLUDE THE STATE OF MISSOURI WHICH SEES BOTH HIGH TEMPS AND HIGH HUMIDITY. DTC CODE P0504. BRAKE LAMP SWITCH HAS ALREADY BEEN REPLACED AND NO ISSUE WITH WI"
— Ford Mustang owner, 05/14/2024
"CAMPAIGN #092 WAS REPLACED ON MY VEHICLE AT BEL AIR HYUNDAI ON 9/14/2010. THE VEHICLE WAS THERE FOR OTHER REGULAR SERVICE AND THE RECALL WAS PERFORMED. AT THAT TIME, THE SWITCH WAS NOT A PROBLEM. NO CHECK ENGINE LIGHT AND WE WERE UNAWARE THAT THIS RECALL WAS EVEN PERFORMED. I'M SURE IT WAS LISTED"
— Hyundai Sonata owner, 08/25/2014
"MY BRAKE LIGHTS NO LONGER FUNCTION. MY "ESC OFF" LIGHT AND CHECK ENGINE LIGHT APPEARED. I HAD A MECHANIC SCAN MY CAR REGARDING THE AFOREMENTIONED ISSUES AND THE CODE THAT DISPLAYED WAS "P0504". AFTER RESEARCHING THIS CODE I HAVE DISCOVERED: THE ENGINE CONTROL MODULE (ECM) HAS DETECTED A FAULT IN "
— Hyundai Sonata owner, 04/20/2015
Source: NHTSA Complaints Database
Most Reported Vehicles for P0504
Based on NHTSA owner complaint data.
| # | Vehicle | Reports |
|---|---|---|
| 1 | HYUNDAI SONATA | |
| 2 | FORD MUSTANG |
P0504 Reports by Vehicle
Detailed NHTSA complaint analysis for each vehicle model.
Diagnostic Tips
- For speed sensor codes: check if speedometer works — if not, output speed sensor is likely bad
- For idle codes: clean throttle body first — carbon buildup is #1 cause
- Check for vacuum leaks with smoke machine
- On electronic throttle vehicles, throttle body relearn may be required after cleaning
Common Mistakes to Avoid
- ❌ Replacing idle air control valve without cleaning throttle body
- ❌ Not performing throttle body relearn after cleaning
- ❌ Ignoring vacuum leaks as cause of idle problems
Frequently Asked Questions
Why does my car idle rough after cleaning throttle body?
Can a bad speed sensor cause transmission problems?
How much does a speed sensor cost?
What To Do Next
Possible Fixes
- 🔧 Replace vehicle speed sensor
- 🔧 Clean throttle body
- 🔧 Replace idle air control valve
- 🔧 Repair vacuum leak
- 🔧 Repair wiring/connector
-
1
Find your vehicle above
Click your make and model for real owner reports and common causes specific to your vehicle.
-
2
Check for recalls
Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.
-
3
Get a professional diagnosis
A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.