The B173D code is rarely a software issue; it is almost always a physical electrical failure. Common causes include:
: If the code persists, a technician using Peugeot-specific software (like Diagbox) may need to perform "telecoding" to re-pair the keys to the BSI. French Car Forum
Before paying expensive diagnostic fees at a dealership, use this logical troubleshooting sequence to isolate and fix the B173D fault code:
A new battery is a great first step, but the issue may still be with the car's systems (fuses, main battery, BPGA, etc.). In some cases, the key itself may need to be re-programmed to the car using a professional diagnostic tool like Diagbox. b173d peugeot work
: A weak car battery drops vehicle voltage during startup, disrupting the data flow between the reader and the BSI.
Using a multimeter, a technician will check for continuity at the brake switch. If the switch does not change state (open/closed) when the pedal is depressed, it is faulty. 3. Replacing the Switch
In simple terms, this code indicates that the Body Control Module (BSI) has detected a discrepancy between the command sent to the door lock and the status signal returned by the door lock mechanism. The BSI commands the door to lock, but the feedback circuit tells the BSI the door is still unlocked (or vice versa). The B173D code is rarely a software issue;
Next comes the . Unlike a mechanical fault, an electrical code like B173D demands a hierarchical approach:
Bad earth connections—for the BSI, the engine ECU (engine management computer), or the ABS (anti‑lock braking system) unit—can cause erratic signals and communication failures. This is especially common on vehicles that have seen corrosion or previous repairs.
This DTC indicates that the BSI (Built-in Systems Interface) or the engine management ECU is unable to verify the transponder chip inside your key, even if the key is inside the car or in the key reader slot. It is a security-related fault code that directly impacts the vehicle's immobilizer system. Common Symptoms message on the dashboard. Engine fails to start (immobilizer activated). In some cases, the key itself may need
The passive antenna coil surrounding the mechanical ignition lock barrel (or the slot reader in hands-free models) is highly susceptible to wear and internal shorts. If the antenna wire degrades, it cannot emit the magnetic field needed to power up and read the key’s electronic transponder chip. 3. Battery Voltage Drops and Electrical Spikes
| | When to Apply | Success Rate | |--------------|------------------|------------------| | Replace the car battery | Battery voltage <12.0 V or battery older than 4–5 years | High | | Replace blown 10A fuse inside BSI | Visual inspection shows a blown fuse | Very high (if fuse is the cause) | | Clean and tighten earth connections | Signs of corrosion or looseness at earth points | Moderate | | Replace key fob battery and re‑synchronise key | Remote locking is intermittent; key detection issues | Moderate | | Perform BSI hard reset | Fault appeared after battery disconnect or low voltage event | Moderate | | Reprogramme keys using Diagbox | Key synchronisation fails; immobiliser remains active | High (if keys are the issue) | | Replace key reader (steering column module) | Diagbox shows no key detection, but keys work on another vehicle | Moderate (less common) | | Update BSI software (dealership) | All other measures fail and BSI is suspected to have a software bug | High (but requires dealer visit) |