P0703 Torque / Brake Switch B Circuit Malfunction

OBD-II Trouble Code - P0703 - Data Sheet

P0703 - Torque Converter/Brake Switch B Circuit Malfunction

What does trouble code P0703 mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to all vehicles since 1996 (Ford, Honda, Mazda, Mercedes, VW, etc.). Although general in nature, the specific repair steps may differ depending on the brand / model.

If you find that a P0703 code has been stored in your OBD-II vehicle, it means that the powertrain control module (PCM) has detected a malfunction in a specific brake switch circuit of the torque converter. This code only applies to vehicles equipped with an automatic transmission.

Automatic transmissions (in mass production vehicles) have been electronically controlled since the 1980s. Most OBD-II equipped vehicles are controlled by a powertrain controller that is integrated into the PCM. Other vehicles use a stand-alone powertrain control module that communicates with the PCM and other controllers via a Controller Area Network (CAN).

A torque converter is a type of hydraulic clutch that connects the engine to the transmission. When the vehicle is in motion, the torque converter allows torque to be transmitted to the transmission input shaft. When the car comes to a stop (when the engine is idling), the torque converter absorbs engine torque using a complex wet clutch system. This allows the engine to idle without stopping.

The lock-up torque converter used in OBD-II equipped vehicles allows the engine to lock on the transmission input shaft under certain conditions. This usually happens when the transmission has shifted to a higher gear, the vehicle has reached a certain speed and the desired engine speed has been reached. In lock-up mode, the torque converter clutch (TCC) is gradually limited until the transmission operates as if it were bolted directly to the engine with a 1: 1 ratio. These gradual clutch limits are known as torque converter lock-up percentage. This system contributes to fuel economy and optimum engine performance. The lock-up of the torque converter is achieved by an electronic solenoid that controls a spring loaded stem or ball valve. When the PCM recognizes the conditions are correct, the lock-up solenoid is activated and the valve allows fluid to bypass the torque converter (gradually) and flow directly to the valve body.

The torque converter lock-up must be disengaged before the engine speed drops to a certain level, and always before the vehicle is idling. Otherwise, the engine will surely stall. One of the specific signals that the PCM looks for when disengaging the torque converter lockup is to press the brake pedal. When the brake pedal is depressed, the brake lever causes the contacts in the brake switch to close, closing one or more circuits. When these circuits are closed, the brake lights come on. The second signal is sent to the PCM. This signal tells the PCM that the brake pedal is depressed and the converter lock-up solenoid should be disengaged.

The P0703 code refers to one of these brake switch circuits. Refer to your vehicle's service manual or all data for specific information on that specific circuit associated with your vehicle.

Symptoms and severity

This code should be considered urgent because serious internal transmission damage can occur if the TCC lock has been inactive for an extended period of time. Most models are designed in such a way that the PCM will disengage the TCC lock and place the transmission control system in limp mode if this type of code is stored.

Symptoms of a P0703 code may include:

• Engine stalls when vehicle rolls to a stop
• TCC lock can be disabled
• Reduced fuel efficiency
• Reduced engine power (especially at highway speeds)
• Unstable gear shifting patterns
• Non-working brake lights
• Stop lights that never turn off and always on
• No torque converter lockup
• Stopping during a stop and in gear due to torque converter lock-up not disengaging.
• Stored DTC
• Illuminated MIL
• Other codes associated with the torque converter, torque converter clutch, or torque converter lockup.

Causes of the P0703 code

This code is usually caused by a faulty or misadjusted brake light switch or a blown fuse in the brake light circuit. Defective brake lamp sockets, burnt out bulbs or shorted, exposed or corroded wiring/connectors can also cause this DTC.

Possible reasons for setting this code:

• Defective brake switch
• Incorrectly adjusted brake switch
• Short circuit or open in wiring and / or connectors in the brake switch circuit marked with the letter B
• Blown fuse or blown fuse
• Faulty PCM or PCM programming error

Diagnostic and repair procedures

A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.

Access the scanner, digital volt / ohmmeter, and service manual (or all data) for your vehicle. You will need these tools to diagnose the P0703 code.

Begin with a visual inspection of the brake light wiring and a general inspection of the wiring under the hood. Check the brake light fuses and replace blown fuses if necessary.

Connect the scanner to the diagnostic connector and get all stored codes and freeze frame data. Make a note of this information as it may help you further diagnose. Clear the codes and test drive the vehicle to see if it resets immediately.

If so: check the battery voltage at the brake switch input circuit using the DVOM. Some vehicles are equipped with more than one brake switch because when the brake pedal is depressed, the brake lights must turn on and the torque converter lock-up must be disengaged. Refer to your vehicle's service manual to determine how your brake switch is configured. If there is battery voltage in the input circuit, depress the brake pedal and check the battery voltage in the output circuit. If there is no voltage on the output circuit, suspect that the brake switch is faulty or incorrectly adjusted.

Additional diagnostic notes:

• Check the system fuses with the brake pedal depressed. Fuses that appear to be OK on the first test may fail when the circuit is under load.
• Often, an incorrectly adjusted brake switch can be mistakenly considered to be faulty.
• For a quick test of TCC operation, bring the vehicle to highway speed (at normal operating temperature), lightly press the brake pedal and hold it down while maintaining speed. If the RPM increases when the brake is applied, the TCC operates and the brake switch releases it properly.
• If the TCC system remains inoperative, serious damage to the transmission can occur.

Common Mistakes When Diagnosing Code P0703

Although the problem with the brake light switch is quite simple, it can be accompanied by other codes that may cause a technician to troubleshoot the torque converter clutch solenoid or wiring.

How serious is the P0703 code?

Code P0703 can cause the brake lights to not work or stay on all the time, which is very dangerous. It may also result in the torque converter not locking up or the lockup circuit not disengaging, which could result in stopping or other drivability issues.

What repairs can fix code P0703?

• Repair, adjustment or replacement of the brake light switch .

Additional comments to consider regarding code P0703

As with other diagnostics, a P0703 code can only point the technician in the right direction. Before replacing any parts, it is important to follow the troubleshooting process in order to correctly diagnose code P0703.

P0703 ✅ SYMPTOMS AND CORRECT SOLUTION ✅ - OBD2 Fault Code

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