Torque Converter/Brake Switch B Circuit Low


The PCM has detected a fault in the brake switch B circuit. The B circuit denotes the stop lamp circuit which deactivates the transmission torque converter lock-up system and cruise control system.

Code Set Parameters

If the PCM detects an abnormal voltage or resistance reading from the brake lamp circuit, a code will be stored and a malfunction indicator lamp may be illuminated. If the ABS controller (or other controller/s) detects the presence of a brake lamp voltage signal, and the transmission control system (usually the PCM) fails to detect the proper amount of voltage or resistance, this type of code will be stored and a service engine soon lamp illuminated. The CAN Bus system is used to transfer data between individual control modules.


These may include inoperative brake lamps or brake lamps that remain illuminated at all times. Other symptoms may include a lack of torque converter lock-up, failure of the torque converter lock-up system to disengage resulting in a stall when coming to a stop, an illuminated service engine soon lamp, and a stored code. Other torque converter clutch or torque converter lock-up codes are also likely to be present.

Common Causes

The most common causes of this type of malfunction code being stored are due to a defective or misadjusted brake lamp switch and a blown stop lamp fuse. Other possible causes include shorted, open, or corroded wiring and/or electrical connectors, blown bulbs, or faulty bulb sockets.

Common Misdiagnosis

Technicians report that the torque converter clutch lock-up solenoid or wiring are frequently condemned in error when the stop lamp switch, lighting controller, or brake lamp circuitry/connectors are at fault.


  • A scanner (or code reader) and a digital volt/ohmmeter will be helpful in successfully diagnosing the potential causes of this code. In most modern OBD-II applications the brake lamp (also called the stop lamp) switch is used to activate exterior brake lamp illumination and deactivate the torque converter clutch system of the automatic transmission (also to deactivate the cruise control, if set)
  • While brake lamp switch design may vary dramatically from one manufacturer to another, it is usually of the “on/off” contact variety and is stationary mounted near the brake foot pedal on the support bracket
  • Some models place the switch directly on the pedal lever and use the pivot point as a fulcrum to move the contacts and open/close the circuit
  • Regardless of design, constant voltage is typically present on one side of the switch and when the contacts in the switch are closed, voltage is transferred across the contacts and out of the switch to one (or multiple applications)
  • Begin your diagnosis 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
  • After the codes are cleared, operate 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
  • If the code immediately returns, check for battery voltage on the input side of the brake lamp switch using the digital volt/ohmmeter
  • If there is no voltage present on the input side of the switch, disconnect any related control modules and check for continuity between the system fuse and the input circuit at the brake lamp switch
  • Repair open or shorted circuits as required
  • If voltage is present on the input side of the brake lamp switch, depress the brake pedal and check for voltage on the output side of the brake lamp switch
  • If you have voltage on the input side and none on the output side (when the brake pedal is depressed) suspect a faulty or misadjusted brake lamp switch
  • Adjust or replace the switch as needed and retest the system afterward to ensure a successful repair. Should you determine that voltage is present on both the input and output sides of the brake lamp switch (with the pedal depressed), visually inspect the exterior lighting to make sure that it works properly
  • If it does not, test the input and output wiring of the turn signal switch for battery voltage when the brake pedal is depressed
  • Refer to the manufacturer’s wiring diagram for wiring color and location. If there is voltage present on the output circuit of the brake lamp switch and the exterior stop lamps operate normally, check circuit continuity and resistance between the brake lamp switch and the PCM (or lighting controller)
  • Again, refer to the vehicle wiring diagram to ensure that all circuits are tested
  • Repair or replace system open or shorted circuits/connectors as required
  • If all circuits coincide with manufacturer’s specifications, suspect a defective PCM
  • Remember that PCM failure is rare and replacement will require reprogramming