What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to all vehicles made from 1996 onwards (Ford, Honda, Mazda, Mercedes, VW, etc.). Although generic, the specific repair steps may vary by make/model.
If you find that a P0703 code has been stored in your OBD-II vehicle, it means the Powertrain Control Module (PCM) has detected a malfunction in a specific torque converter brake switch circuit. This code only applies 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 standalone transmission control module that communicates with the PCM and other controllers via the Controller Area Network (CAN).
The torque converter is a type of hydraulic coupling that connects the engine to the transmission. When the vehicle is moving, the torque converter allows torque to be transferred to the transmission input shaft. When the vehicle stops (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 lock-up torque converter, used in OBD-II-equipped vehicles, allows the engine to lock to the transmission input shaft under certain conditions. This typically occurs when the transmission has shifted to the highest gear, the vehicle has reached a certain speed, and the desired engine RPM level has been achieved. In lock-up mode, the Torque Converter Clutch (TCC) is gradually engaged until the transmission operates as if it were bolted directly to the engine in a 1:1 transfer ratio. These gradual clutch engagements are referred to as torque converter lock-up percentage. This system promotes energy efficiency and optimal engine performance. Torque converter lock-up is achieved using an electronic solenoid that controls a spring-loaded spool valve or rod. When the PCM recognizes that conditions are correct, the lock-up solenoid is activated, and the valve allows fluid to bypass (gradually) the torque converter and be applied directly to the valve body.
The torque converter lock-up must be disengaged before engine RPM levels drop to a certain point and definitely before the vehicle comes to a stop with the engine idling. Otherwise, the engine will undoubtedly stall. One of the specific signals the PCM looks for to disengage the torque converter lock-up is the application of the brake pedal. When the brake pedal is applied, the brake lever arm causes contacts in the brake switch to close, completing one or more circuits. When these circuits are completed, the brake lights are illuminated. A second signal is sent to the PCM. This signal tells the PCM that the brake pedal has been pressed and that the torque converter lock-up solenoid should be disengaged.
The P0703 code relates to one of these brake switch circuits. Consult your vehicle’s service manual or AllData for specific information about this particular circuit as it pertains to your vehicle.
Symptoms
and severity
This code should be treated as urgent because severe internal transmission damage could result if the TCC lock-up is inoperative for an extended period. Most models are designed so that the PCM disables the TCC lock-up and places the transmission control system in limp mode if such a code is stored.
Symptoms of a P0703 code may include:
Engine stalls when the vehicle stops
TCC lock-up may be disabled
Reduced fuel efficiency
Decreased engine performance (especially at highway speeds)
Irregular transmission shifting patterns
No brake light illumination
Causes
Possible causes for this code being set are:
Faulty brake switch
Misadjusted brake switch
Shorted or open wiring or connectors in the designated brake switch circuit B
Blown fuse or burned fuse link
PCM programming error or faulty PCM
Diagnostic and Repair Procedures
A good starting point is always to check the Technical Service Bulletins (TSBs) for your specific vehicle. Your issue may be a known problem with a known fix published by the manufacturer and can save you time and money during diagnosis.
Gather a scan tool, a digital volt/ohmmeter, and a service manual (or AllData) for your vehicle. These tools will be necessary to diagnose a P0703 code.
Start with a visual inspection of the brake light wiring and a general inspection of the wiring under the hood. Test the brake light system fuses and replace any blown fuses as needed.
Connect the scan tool to the diagnostic connector and retrieve all stored codes and freeze frame data. Take note of this information as it may be helpful for further diagnosis. Clear the codes and test-drive the vehicle to see if the code resets immediately.
If it does: Check for battery voltage at the brake switch input circuit,
Using the DVOM. Some vehicles are equipped with multiple brake switches because, when the brake pedal is pressed, the brake lights must activate and the torque converter lock-up must disengage. Consult your vehicle’s service manual to determine your brake switch configuration. If there is battery voltage at the input circuit, press the brake pedal and check for battery voltage at the output circuit. If there is no voltage on the output circuit, suspect a faulty or misadjusted brake switch.
Additional Diagnostic Notes:
Check the system fuses with the brake pedal depressed. Fuses that may seem operational during an initial test can fail when the circuit is under load
Often, a misadjusted brake switch can be misdiagnosed as faulty
To quickly test TCC operation, bring the vehicle to highway speed (at normal operating temperature), lightly press and hold the brake pedal while maintaining speed. If the RPM level increases when the brake is applied, the TCC is operational and the brake switch is disengaging it correctly
Significant transmission damage can occur if the TCC system remains inoperative