P0504: Brake Switch A/B Correlation
A generic powertrain (engine and transmission) trouble code, same meaning on every brand.
Yes with care - the two brake signals disagree, and systems that need certainty will shut off.
Symptoms
- Cruise control inoperative
- ABS or stability light in some cases
- Brake lights working but codes returning
Likely causes, most common first
- One of the two brake switch contacts worn
- Misadjustment so the contacts trip at different points
- Connector pin corroded
- Wiring fault on one signal
Read P0504 yourself before paying for diagnosis
A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.
What is going on and how it gets diagnosed
P0504 exists because carmakers stopped trusting a single brake signal: the switch carries two contacts, often one normally-open and one normally-closed, and the computer checks that they always disagree in the healthy way. This code means they briefly agreed - which should be impossible - so one contact is lying.
A correlation fault like this is subtler than stuck-on or stuck-off: the brake lights may work perfectly while cruise control refuses duty, because cruise demands both signals be plausible before it will engage. The repair is the same inexpensive switch; the insight is knowing that working brake lights do not clear the switch of suspicion.
Repair options and cost
Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.
Owner reports mentioning P0504
“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 wiring. Positive the plastic stop bumper is the issue.”2015 Ford Mustang · May 2024
“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 correlation between the two brake switch circuits. This very problem has resulted a massive recall for hyundai & kia vehicles. Unfortunately, my vehicle is not eligible for the recall according to the VIN look up for this recall campaign (110). I should not have to pay out of pocket for this manufacturer defect!!! *tr”2007 Hyundai Sonata · Apr 2015 · 82,771 mi
“Esc off light on dash then check engine. Code is p0504 for brake switch. This condition is covered under other hyundai campaigns and recalls but not for our VIN / build date. Please consider working with hyundai to provide resolution to this issue. We are also being told there is a two week wait to have this repair performed. Vehicle safety during this delay is a concern should the esc system malfunction while driving. Thank you. *tr”2007 Hyundai Santa FE · Jun 2013 · 85,000 mi
Show 1 more owner reports
“Have had occurrences of 'check engine' light come on and the code that came up was p0504 indicating a brake switch malfunction. After several trips with the car, the 'check engine' light goes off, but reoccurs at a later date. *tr”2009 Hyundai Azera · Jun 2013 · 85,122 mi
Cars that report P0504 most often
Based on mentions of this code in NHTSA owner complaints.
Frequently asked questions
Can I drive with P0504?
Yes with care - the two brake signals disagree, and systems that need certainty will shut off.
How much does it cost to fix P0504?
Typical US repair costs range from $40 for the simplest fix (brake light switch) to $180 in the worst case. The right number depends on which cause your car has - see the causes list.
Related codes
The codes closest to P0504. They often appear together, and telling them apart is usually what narrows the diagnosis.
Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.