Torque Converter/Brake Switch B Circuit Malfunction
The ECU detected a problem with the brake switch circuit that tells the transmission when to lock or unlock the torque converter. Think of it like a communication line between your brake pedal and transmission—if that line is broken or sending bad signals, the transmission gets confused about when to engage.
The ECU monitors the brake switch circuit voltage to determine brake pedal status, which signals whether the torque converter should lock for efficiency or unlock for smooth operation. The circuit should toggle between high and low voltage states when brakes are applied and released. If voltage remains stuck, fluctuates erratically, or fails to respond to brake input, the ECU logs a fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Brake Switch Voltage (Park/Neutral) | 0V or 12V (stable, state-dependent) | Floating voltage, no transition, or stuck high/low |
| Circuit Continuity | >10 kΩ or <1 kΩ (depending on state) | Open circuit or short to ground |
Code P0704 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0704 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.