Technical Description of OBD-II Trouble Code
Malfunction in the Circuit for Torque Converter/Brake Switch B
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all vehicles manufactured from 1996 onwards (Ford, Honda, Mazda, Mercedes, VW, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
If you find that a code P0703 has been stored in your OBD-II vehicle, it means that the powertrain control module (PCM) has detected a malfunction in a specific circuit related to the torque converter brake switch. This code is only applicable to vehicles equipped with automatic transmissions.
Automatic transmissions in mass-produced vehicles have been electronically controlled since the 1980s. Most OBD-II equipped vehicles are managed by a transmission controller integrated into the PCM. Other vehicles use a separate transmission control module that communicates with the PCM and other controllers via the controller area network (CAN).
The torque converter is a type of fluid coupling that connects the engine to the transmission. When the vehicle is in motion, the torque converter transfers torque to the transmission input shaft. When the vehicle comes to a stop with the engine idling, the torque converter absorbs the engine’s torque using a complex system of wet clutches. This allows the engine to idle without stalling.
The lockup torque converter, used in OBD-II equipped vehicles, allows the engine to lock into the transmission input shaft under certain conditions. This typically occurs when the transmission has shifted into the highest gear, the vehicle has reached a certain speed, and the desired engine RPM level has been achieved. In lockup mode, the torque converter clutch (TCC) gradually restricts until the transmission functions as if it were directly connected to the engine in a 1:1 transfer ratio. These gradual clutch restrictions are referred to as the percentage of torque converter lockup. This system promotes fuel efficiency and optimal engine performance. Torque converter lockup is achieved using an electronic solenoid that controls a spring-loaded rod or ball valve. When the PCM recognizes that conditions are correct, the lockup solenoid is activated, allowing fluid to bypass the torque converter gradually and be applied directly to the valve body.
Torque converter lockup must be disengaged before engine RPM levels decrease to a certain level and definitely before the vehicle comes to a stop with the engine idling. Failure to do so will result in the engine stalling. One of the specific signals that the PCM looks for in torque converter lockup disengagement is the application of the brake pedal. When the brake pedal is applied, the brake lever arm closes contacts in the brake switch, completing one or more circuits. When these circuits are completed, the stop lamps illuminate, and a second signal is sent to the PCM. This signal informs the PCM that the brake pedal has been depressed and the torque converter lockup solenoid needs to be disengaged.
The P0703 code relates to one of these brake switch circuits. Consult your vehicle service manual or All Data for specific information about this particular circuit as it relates to your vehicle.
Symptoms & Severity
This code should be treated as urgent because prolonged inoperability of the TCC lockup can result in severe internal transmission damage. Most models are designed so that if a code of this type is stored, the PCM will disable TCC lockup and place the transmission control system in limp-in mode.
Symptoms of a P0703 code may include:
Causes
Possible causes for this code to be set are:
Diagnostic and Repair Procedures
A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known problem with a known solution provided by the manufacturer, which can save you time and money during diagnosis.
Obtain a scanner, a digital volt/ohmmeter, and a service manual (or All Data) for your vehicle. These tools will be necessary to diagnose a code P0703.
Begin with a visual inspection of the brake lamp wiring and a general inspection of the underhood wiring. Test the fuses for the stop lamp system and replace any blown fuses as necessary.
Connect the scanner to the diagnostic connector and retrieve all stored codes and freeze frame data. Make a note of this information as it may assist you in further diagnosis. Clear the codes and test drive the vehicle to see if the code immediately reappears.
If it does: Check for battery voltage at the input circuit of the brake switch using the DVOM. Some vehicles are equipped with multiple brake switches because, when the brake pedal is depressed, the stop lamps must be activated and the torque converter lockup must be deactivated. Consult your vehicle service manual to determine how your brake switch is configured. If there is battery voltage on the input circuit, depress the brake pedal and check for battery voltage on the output circuit. If there is no voltage on the output circuit, suspect a defective or misadjusted brake switch.
Additional diagnostic notes:
