P0504 OBD-II Trouble Code: Brake Switch “A”/”B” Correlation

Our certified mechanics come to you · 12-month, 12,000-mile warranty

SCHEDULE P0504 DIAGNOSTIC Get a fair and transparent estimate upfront
Red-stars EXCELLENT RATING ON

Cost of diagnosing the P0504 code

P0504 is a diagnostic trouble code (DTC) for "Brake Switch “A”/”B” Correlation". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $70.00. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $30.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

Cars Estimate Credit towards follow-up repair Earliest Availability
Ford$70.00$30.00
Toyota$70.00$30.00
BMW$70.00$30.00
SCHEDULE P0504 DIAGNOSTIC Get a fair and transparent estimate upfront

Check Engine Light

Diagnostic Trouble Code (DTC): P0504

P0504 code definition

When a malfunction is detected in the brake light switch of a vehicle, the PCM (powertrain control module) records the P0504 code, and the Check Engine Light is illuminated.

What the P0504 code means

If a failure occurs in the circuit for the brake light, a number of problems can occur. To keep you out of danger this code will be recorded, and the Check Engine Light will remain on until the cause of the problem is resolved and the code is cleared from the PCM.

What causes the P0504 code?

A number of issues can cause the P0504 code to be recorded, but fortunately most of them are fairly simple and easy to diagnose. These include:

  • A blown or malfunctioning brake light fuse
  • A damaged or faulty brake light switch
  • A blown brake light bulb
  • A shorted or open brake switch harness
  • A problem with the electrical connection or circuit for the brake switch

What are the symptoms of the P0504 code?

The first indication that your PCM has stored a P0504 code will likely be an illuminated Check Engine Light. Aside from this, you may also notice other symptoms, including:

  • Pressing the brake pedal doesn’t engage or disengage the vehicle’s cruise control system.
  • One or both brake lights will not turn on when pressing the brake pedal.
  • One or both brake lights remain on, even after taking your foot off of the brake pedal.
  • Depressing the brake pedal at highway speeds causes the engine to stall.
  • The shift interlock system fails to work properly.

How does a mechanic diagnose the P0504 code?

After identifying the P0504 code with an OBD-II scanner, the mechanic will clear the code and then perform an inspection of all possible causes for the code. They will start by testing the brake light fuse, and then they will test and/or replace the brake light bulb. If these measures do not fix the problem, they will move on to test the brake switch itself, as well as its wiring and connectors.

Common mistakes when diagnosing the P0504 code

Often, when a brake light does not turn on when the driver depresses the brake pedal, they will assume that the problem is a burnt out bulb. They may then replace the bulb and find that this does nothing to solve the problem. If there is an underlying problem with the brake switch or circuit, replacing a blown brake fuse may also be a mistake, as the root problem will likely cause the fuse to blow again.

How serious is the P0504 code?

It’s very dangerous when brake lights do not turn on and off when depressing or releasing the brake pedal. Following traffic cannot tell if you intend to slow down or if you need to stop suddenly, and an accident can easily occur. Likewise, if depressing the brake pedal does not disengage the cruise control system, you may find yourself in another dangerous situation. Thus you can see that the P0504 code is very serious, and should be dealt with immediately.

What repairs can fix the P0504 code?

In most cases, repairing the cause of a P0504 code is fairly simple. Depending on what the root problem is, some of the most common repairs include:

  • Replacing a burnt out brake light bulb.
  • Repairing or replacing wires or connectors for the brake switch harness or circuit.
  • Replacing the brake switch.
  • Replacing a blown brake light fuse.

Additional comments for consideration regarding the P0504 code

In addition to causing potentially dangerous situations on the road, a P0504 code can also result in a failed emissions test. Though the brake light switch does not have a direct effect on the vehicle’s emissions, it will illuminate the Check Engine Light, which will cause the vehicle to fail an OBD-II emissions test.

Need help with a P0504 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

pcm
trouble codes
Check Engine Light
Brake Lights

Fast and easy Check Engine Light is on Inspection service at your home or office.

SCHEDULE P0504 DIAGNOSTIC
Get a fair and transparent estimate upfront

Recent Check Engine Light is on Inspection reviews

Excellent Rating


YourMechanic Check Engine Light is on Inspection Service

Average Rating

4.5/5

Number of Reviews

3,203

Rating Summary
2,919
140
39
22
83
2,919
140
39
22
83

Nathan

26 years of experience
198 reviews
Nathan
26 years of experience
Nathan was knowledgeable about my car and very educational as well. He explained everything he was doing and why. I admire his attention to detail. He really showed genuine interest and attentiveness to the problem. He’s a great conversationalist and was easy to work with.
2013 CHEVROLET CRUZE - CHECK ENGINE LIGHT IS ON - GLENPOOL, OKLAHOMA

Mack

19 years of experience
72 reviews
Mack
19 years of experience
Very nice and knew what he was doing. Got what I needed done even though was told wrong by the office.
1999 FORD F-150 - CHECK ENGINE LIGHT IS ON - MIDLOTHIAN, TEXAS

Scott

31 years of experience
24 reviews
Scott
31 years of experience
Very nice friendly mechanic. Very helpful with with questions that I had. Offered good advice. He explained some of the problems to me and what should be the next to resolve it before spending unnecessary monies. This was my first time using him and will not be my last. I would recommend him to anyone. Especially women because he does not seem to take advantage of our knowledge of not knowing things about cars.
2001 HYUNDAI ELANTRA - CHECK ENGINE LIGHT IS ON - PARKVILLE, MARYLAND

Tyrone

6 years of experience
61 reviews
Tyrone
6 years of experience
He showed up on time and got my diagnostic done quickly. He also gave me honest recommendations on what I need to do for my car.
2008 HONDA ACCORD - CHECK ENGINE LIGHT IS ON - CULVER CITY, CALIFORNIA

More related articles

Veteran and Military Driver Laws and Benefits in New Mexico
The state of New Mexico offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...
Auto Safety Tips
Driving is more than a way to get from point A to point B. Owning and driving a car can also be a highly enjoyable experience. Whether a person is driving...

Related questions

Q: i cannot find a used engine for my truck,will a engine from a grand marquis fit in the truck?

I looked up the two applications. Those two engines use completely different castings for both the heads and the block and the flywheel (flex plate) is different as well which will make it incompatible with your transmission. If you search...

Q: Misfire caused bad o2 sensor?

As you may know the oxygen sensors which are part of your emissions system read exhaust gases to determine whether or not the appropriate amount of oxygen is present in the exhaust. Based upon this reading, this information is relayed...

Q: Check Engine Light is on - code P0126

Hello there. Many common faults will cause the P0126 Code with your 2008 Mazda 3. The P0126 code meaning is "Insufficient Coolant Temperature for Stable Operation." This is telling us the car is reading a coolant temperature that is below...

How can we help?

Our service team is available 7 days a week, Monday - Friday from 6 AM to 5 PM PST, Saturday - Sunday 7 AM - 4 PM PST.

1-800-701-6230 · hi@yourmechanic.com