Error Code P0703 is defined as Torque Converter/Brake Switch B Circuit Malfunction. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. This includes vehicle models from but not limited to Ford, Honda, Mazda, Mercedes Benz, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model, and powertrain configuration.
If the vehicle shows up this error code, this means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined a problem in a certain torque converter brake switch circuit. This type of code is exclusive to vehicles with automatic transmission.
Since the 1980s, vehicles equipped with automatic transmission are controlled electronically. Most vehicles with the OBD-II system are managed by the transmission controller, a system integrated to the PCM. Other vehicles, however, use stand-alone TCM (transmission control module) which communicates to the PCM and other controllers through the CAN (controller area network).
The torque converter works like a fluid coupling that links the engine to the transmission. When the vehicle moves, the torque converter allows torque to transfer to the transmission input shaft. When the vehicle stops (while the engine is at idle), the torque converter absorbs the torque from the engine, with the help of complex systems of wet clutches. This allows the engine to idle without stalling.
OBD-II vehicles use a lockup torque converter. This allows the engine to lock into the transmission input shaft under certain conditions. This usually happens when the transmission shifts into the highest gear, the vehicle reaches a certain speed and the desired engine RPM. At lockup mode, the TCC (torque converter clutch) is gradually restricted until the transmission is working, as though it were bolted straight to the engine in a 1:1 transfer ratio. This gradual restrictions in the clutch are known as the percentage of torque converter lockup, a system that promotes fuel efficiency and optimal engine performance. This is achieved using electronic solenoid that controls a spring-loaded rod, or ball valve. When the PCM recognizes the right conditions for this, the lockup solenoid is quickly activated, and the valve allows fluid to gradually bypass the torque converter, and apply it straight to the valve body.
The torque converter lockup must be disengaged before the engine RPM levels decline to a certain level, and most definitely before the vehicle comes to a full stop, with the engine idling. If it’s unable to do so, the engine will stall. One of the specific signals that the PCM looks for in torque converter lockup disengagement is the use of brake pedal. When the brake is applied, a brake lever arm will come in contact to the brake switch and closes it, completing one or more circuit/s. When these circuits are complete, the stop lamps will light up. A second signal is then sent to the PCM, which will let it know that the brake pedal has been depressed, and the torque converter lockup solenoid must be disengaged.
Error Code P0703 specifies to one of these brake switch circuits. Thus, make sure you consult with the vehicle’s service manual (or similar) for specific information regarding this particular circuit as it relates to the vehicle.
- Engine stalls when the vehicle rolls to a stop
- A decrease in engine performance (especially at highway speed)
- Increase in fuel consumption
- Disabled TCC lockup
- Erratic transmission shifting
- No activation of the stop lamp
Some of the common causes of this code include:
- Faulty or misadjusted brake switch
- Shorted or open connectors or wiring at brake switch circuit designated as B
- Burnt fusible link
- Blown fuse
- Faulty or programming error in PCM
How to Check
As with most codes, checking with a TSB (technical service bulletin) is a good starting point to diagnose this code. Other things you need include the basics, such as a scanner, DVOM (digital volt/ohmmeter), transmission gauge, and vehicle service manual (or equivalent).
Start the diagnosis by visually inspecting the clutch switch circuit wiring. Test all system fuses and replace any blown fuse. Next, load test the battery, then check the battery cables. Check the alternator too.
Start inspecting the brake lamp wiring and general underhood wiring. Then, test the stop lamp system fuses. Replace any blown fuse as necessary.
Find the diagnostic connector, connect your scanner and retrieve all stored codes, including their freeze frame data. Take note of this information. Then, clear the codes and then take the vehicle for a test drive to see if the code comes back immediately.
If the code resets, then use the DVOM to check the battery voltage at the input circuit of the brake switch. Some vehicles may be equipped with multiple brake switches so when the brake pedal is depressed, the stop lamp is activated, and torque converter lockup is deactivated. Make sure to consult with your vehicle service manual to determine how the brake switch is specifically configured. If the circuit input has battery voltage, depress the brake pedal and check the battery voltage on the output circuit. If there’s no voltage on the output circuit, then you can suspect a defective or misadjusted brake switch.
- Make sure to check the fuses with the brake pedal depressed. Fuses may look operational at first, but fail when the circuit is loaded.
- Misadjusted brake switch is a common misdiagnosis for this code.
- To test the TCC operation quickly, bring the vehicle to highway speed (at normal operating temperature), and then lightly tap the brake pedal. Hold it while maintaining the speed. If the RPM level increases when the brake pedal is depressed, then your TCC is operational, as the brake switch is deactivating it properly.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Repair, replacement, and readjusting of stop lamp switch
- Replacement of blown fuse or fusible link
- Repair or replacement of open or shorted connector and/or wiring at brake switch B circuit
This code is considered as urgent, as it pertains to severe problem in the transmission, which leads to TCC lockup over a long period of time. Most vehicle models are actually designed so that PCM will disable TCC lockup, and put the transmission control system in limp-in mode if this type of code is stored.
Major damages to the transmission can happen when the TCC system remains defective.
When Error Code P0703 is present, this can lead to brake lamps not working properly, or staying lit all the time, which can be very dangerous.