Error Code P0504 is defined as Brake Switch A/B Correlation Code. This means there’s a malfunction detected in the vehicle’s brake light switch. This error code is a generic code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made in 1996 up to present. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

When Error Code P0504 is set, that means the vehicle’s computer system or PCM (powertrain control module, also known as ECM or engine control module) has detected a failure in the brake light circuit. The PCM monitors all circuits in the vehicle and checks for any abnormalities, such as lack of voltage or out of range conditions.

The brake light switch is connected to multiple circuits, which can result in dangerous situations. The brake switch itself is made up of two signal outputs. If there’s a malfunction in the switch, the Check Engine light will activate, and the Error Code will register to warn you of the danger. This is a safety factor that demands immediate repair.

Common Symptoms

As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. Other possible symptoms may include:

  • Brake light will continuously light up or fail to light up when the pedal is depressed
  • Difficulty or impossible to shift out of park
  • Vehicle stalls when brakes are depressed while at cruising speed
  • Cruise control will not activate
  • Failure of shift interlock system

Possible Causes

There can be many possible causes of this code, but fortunately, they are fairly easy to diagnose. These causes include:

  • Defective brake light switch
  • Worn out brake light switch
  • Blown of defective brake light fuse
  • Shorted or open brake switch harness
  • Blown brake light bulb
  • Electrical connection problem or circuit problem in the brake switch
  • Defective PCM (rare)

How to Check

After verifying the existence of the code using an OBD-II scanner, mechanics will clear the codes before performing a visual inspection of all the possible causes of the code.

They will start with the brake light fuse and test the brake light bulb. Defective or damaged brake light bulbs are replaced.

If this still doesn’t fix the problem, mechanics will proceed on testing the brake switch, its wirings, and connectors.

If the brake lights are working properly, but the code persists, mechanics will heck the remaining wires on the brake light switch and then remove the connector to probe the wires for power. With the pedal depressed, they will back probe the wire adjacent to the power wire to see if it’s good. If there is no power, then the switch must be replaced.

If there is power when the pedal is depressed, then the problem could be in the wiring to the computer or the computer itself. In this case, they will have to locate and back probe the STP terminal on the computer ground. If there is 12V on the voltmeter, that means the computer has gone bad. Which means it must be replaced.

If there is little to no voltage, then the harness that connects the computer to the switch must be repaired or replaced.

How to Fix

Depending on the root of the problem, diagnosis and repairs for this code are mostly straightforward. Common repairs include:

  • Replacement of burnout brake light bulb
  • Replacement of the brake switch
  • Repair or replacement of connectors or wires for the brake switch circuit or harness
  • Replacement of blown brake light fuse

Many times when the brake light doesn’t work when the brake pedal is depressed, people assume the problem is caused by a burn out bulb and replace it. However, underlying problems with the brake switch or circuit may be what’s causing the problem, and simply replacing the bulb may cause the fuse to blow again. Thus, it’s a thorough diagnosis is important.

It’s extremely dangerous when the brake lights don’t work properly, as the following traffic will have difficulty anticipating your movements, which may lead to collision and serious accidents.  Also, when the brake pedal doesn’t work, it can put you in a very dangerous situation. This is why it’s extremely important to address this code as soon as possible.