Brake Switch "A"/"B" Correlation


The PCM has detected a malfunction in the stop lamp circuit.

Code Set Parameters

The brake switch controls brake light activation, cruise control deactivation, and shift interlock functions. If brake lamp circuit voltage is abnormal, the PCM will recognize the failure, store a code, and illuminate the service engine soon light.


The cruise control system may fail to engage or disengage when the brake pedal is depressed, the brake lamps (on the rear of the vehicle) may either remain illuminated or fail to illuminate, the shift interlock system may fail to operate properly, or the engine may stall when the brakes are depressed at highway speeds.

Common Causes

The most common cause of this malfunction is a blown brake lamp fuse. This may be caused by a faulty brake lamp switch or an improperly connected trailer harness. Other possibilities may include open or shorted electrical wiring or connectors, faulty brake lamp bulbs, or faulty bulb sockets in the tail lamp assemblies.

Common Misdiagnosis

Brake lamp bulbs are often replaced in an unsuccessful attempt to repair this condition. Technicians also report that fuse replacement is only a temporary fix as a faulty brake lamp switch or wiring malfunction is at the root of the problem.


  • The most common type of brake lamp switch is a contact switch that is activated by the movement of the brake pedal mechanism
  • As the pedal is depressed, a push pin in the switch acts as a cam
  • This releases a set of spring loaded contacts, allowing them to touch and complete the circuit
  • Another design utilizes a rocker switch, with a metal plate that moves slightly back and forth as the brake pedal is depressed, to complete the circuit. Typical failures involve worn or broken brake lamp switches in high mileage vehicles
  • After the brake lamp switch is replaced, some models require adjustment
  • If the brake lamp switch is not properly adjusted, a code can possibly be set. Several tools will be instrumental in successfully diagnosing this code
  • A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • Continue your diagnosis by depressing the brake pedal with a “dead-man” tool or prybar
  • Move to the rear of the vehicle and observe brake lamp operation
  • If any of the brake lamps are illuminated, you have a brake lamp bulb problem
  • Replace bulbs as required, reset the code, and retest brake lamp operation
  • If no brake lamps are illuminated, check the brake lamp (may be labeled “stop lamps”) fuse and replace as needed
  • If the fuse is blown, reset the code and retest brake lamp operation to see if the fuse blows again
  • If the fuse does not blow immediately, the vehicle may need to be test driven over a period or hours or even days
  • Moisture inside of the tail lamp housings may result in a blown fuse or blown bulb
  • If the fuse immediately blows, you may utilize a short finder to locate shorted or open circuits
  • If a short finder is not available, carefully inspect brake lamp circuit wiring and connectors for signs of damage or corrosion and replace components as required
  • You may also replace the fuse and disconnect sections of the brake lamp switch circuit individually in order to eliminate specific sections and isolate the short
  • Should you find the fuse in good working order, you will want to continue your diagnosis with the brake lamp switch, itself
  • Gain access to the switch, located underneath the driver’s side instrument panel and find the brake lamp switch electrical connector
  • Using the digital volt/ohmmeter, check for input voltage to the brake lamp switch
  • If voltage is detected, depress the brake pedal and check for output voltage from the brake lamp switch
  • If the switch is operating properly from a mechanical standpoint and there is no output voltage signal but there is input voltage, replace the brake lamp switch, reset the code, and retest the system
  • If brake lamps remain illuminated constantly, you may test brake lamp switch operation in a similar manner, only reversed
  • If there is a voltage output signal from the brake lamp switch, but no brake lamps are illuminated, continue your diagnosis by testing for voltage through the turn signal switch
  • If output voltage is present from the turn signal switch, trace system wiring to the rear of the vehicle
  • Use the digital volt/ohmmeter to check continuity in the circuit and replace wiring, connectors, or components as required.