P0703 OBD-II Trouble Code: Torque Converter/Brake Switch B Circuit Malfunction

P0703 code definition

A P0703 code means the PCM has registered a fault in the brake switch B circuit. This is the dedicated stop lamp circuit, which also deactivates the cruise control system and transmission torque converter lockup system, as well as the stop lamps.

What the P0703 code means

With a P0703 code, the PCM has registered a voltage or resistance reading from the brake lamp circuit that’s out of normal spec, triggering a hard code and an illuminated malfunction indicator lamp (MIL). The ABS controller is what passes brake lamp voltage information on, sometimes using the CAN bus system to transfer data between individual modules. When the transmission control system (usually in the PCM) gets an unusual reading on brake lamp voltage and resistance, this code will be stored.

What causes the P0703 code?

Commonly, this code is due to a failed or misadjusted brake lamp switch or blown fuse in the stop lamp circuit. Faulty brake bulb sockets, blown bulbs or shorted, open or corroded wiring/connectors can cause this fault code as well.

What are the symptoms of the P0703 code?

• Inoperative brake lamps
• Brake lamps that never shut off and are always illuminated
• Failure of the torque converter to lock up
• Stalling while stopped and in gear, due to failure of the torque converter lockup to disengage
• Stored trouble code
• Illuminated MIL
• Other codes related to torque converter, torque converter clutch or torque converter lockup

How does a mechanic diagnose the P0703 code?

For most OBD-II equipped vehicles , the stop lamp switch also deactivates the torque converter clutch and cruise control, as well as engaging brake lamp illumination. Most stop lamp switches are of the on/off variety and are mounted on the brake pedal’s support bracket. Designs can vary greatly – some models have the switch mounted on the pedal lever itself, using the lever’s pivot point as the fulcrum to move contacts, opening and closing the circuit.

In any design, the constant voltage is present on one side of the switch; when the switch’s contacts are closed, voltage moves across the switch’s contacts and on to another application. Your diagnosis will begin with a visual inspection of all wiring and connectors. Repair/replace any damaged, disconnected, shorted or corroded wiring, connectors or components as needed. Retest the system after repairs.

For further diagnosis, you’ll need a manufacturer’s wiring schematic, OBD-II scanner or code reader and a digital volt/ohmmeter. Inspect the PCM’s wiring and connectors and repair any damaged, disconnected, shorted or corroded wiring, connectors or components as needed. Retest the system. If all wiring and components seem to be in good working order, connect the code reader to the diagnostic port and record any stored codes. Clear the codes and test drive the vehicle to see if the code registers again.

If the code doesn’t register again, you may have an intermittent condition which might need to be allowed to worsen before a correct diagnosis and repair can be made. If the code immediately registers again:

• Check for battery voltage on the stop lamp switch’s input side, using the volt/ohmmeter

• If no voltage can be found on the input side, disconnect any related control modules and check for continuity between the system fuse and stop lamp switch input circuit. Repair open or shorted circuits as needed.

• If voltage is present on the stop lamp switch’s output side, check for voltage on the output side with the brake pedal depressed.

• If you find voltage on the input side but none on the output side (with brake pedal depressed), the problem may be a failed or misadjusted stop lamp switch. Most stop lamp switches are adjustable – adjust or replace the switch and retest the system.

• If voltage is present on the input and output sides of the stop lamp switch, with the brake pedal depressed, check the brake lamps to see if they are working properly. If not, test the turn signal switch for battery voltage while brake pedal is depressed. Test input and output sides of the turn signal switch. Refer to a factory wiring diagram for proper wiring color and location.

• If everything else checks – voltage present at output circuit of the stop lamp switch and turn signal switch, and brake lamps are working properly – check circuit continuity and resistance between the stop lamp switch and PCM. Be sure to check all circuits, using vehicle wiring diagram. Repair/replace any open or shorted circuits or connectors.

• If no other problems are apparent, suspect a defective PCM but remember that a failed PCM is rare and will require reprogramming to replace.

Common mistakes when diagnosing the P0703 code

While a problem with a stop lamp switch is fairly simple, it can be accompanied by other codes that can lead a technician to troubleshoot the torque converter clutch lockup solenoid or wiring.

How serious is the P0703 code?

A P0703 code can lead to brake lamps not working or staying lit all the time, which is very dangerous. It can also lead to the torque converter failing to lock up, or the lockup circuit failing to disengage, which can lead to stalling problems or other drivability issues.

What repairs can fix the P0703 code?

• Repair, adjustment or replacement of the stop lamp switch.

Additional comments for consideration regarding the P0703 code

As with other diagnostics, a P0703 code can only point a technician in the right direction. It’s important to follow a process of elimination in order to reach a correct diagnosis for a P0703 code, before replacing any parts.

Need help with a P0703 code?

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