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For models requiring physical insertion, the transponder coil reader surrounding the ignition switch can fail. In keyless entry models, Low Frequency (LF) interior antennas can become disconnected or suffer from water damage, blinding the car to the key's presence. 3. Low Vehicle Battery Voltage

Note: Always disconnect the vehicle battery before attempting any electrical repairs to avoid short circuits or airbag faults.

The physical slot where you insert the key can become dirty, dusty, or have bent pins, preventing communication.

+------------------------+ Enters Cabin +-------------------------+ | Smart Key / Remote | = = = (Wireless RF / LF) = = > | Proximity Antenna/Slot | | (Transponder Chip) | | (Ignition Barrel Coil) | +------------------------+ +------------+------------+ | | (Hardwired Link) v +------------------------+ Unlocks Starting +------------+------------+ | Engine Control ECU | < = = = (CAN-Bus Signal) = = = | Central BSI Module | | (Immobilizer Status) | | (Decodes Fault B173D) | +------------------------+ +-------------------------+ Primary Root Causes 1. Key Fob Battery Depletion or Signal Interference

The Peugeot fault code identifies a failure in the recognition of the mechanical or electronic key (or remote control) by the vehicle's immobiliser system. This diagnostic trouble code (DTC) is commonly associated with Peugeot and Citroën vehicles, particularly when the car fails to start or displays a "key not detected" warning. Understanding B173D: Causes and Symptoms

: Issues with the transponder aerial (the ring around the ignition barrel) or associated wiring can disrupt the signal . Recommended Steps for Resolution

: If a remote control button is pressed repeatedly out of range of the vehicle, its rolling code counter can lose alignment with the BSI memory.

Before attempting complex diagnostics, replace the battery in the key fob, even if it seems to work for locking the doors. 2. Try the Spare Key

While rare for a single cylinder, some BlueHDi engines have a master fuse for the glow plug control unit. If this fuse blows, you will get multiple glow plug codes for all cylinders (B173C, B173D, B173E, B173F). If you only have B173D, the fuse is fine.

The most common and easiest fix; a weak battery can prevent the hands-free signal from being strong enough for the reader. The "BSI Glitch":

As we dig deeper, several theories emerge:

The engine refuses to crank or turn over because the immobilizer cuts off fuel delivery or starter motor activation.

Using your multimeter, probe the wiring harness connector (sensor disconnected):

Reconnect the battery terminal. Wait without touching anything inside the vehicle.

conditions where the vehicle turns over but refuses to fire. Common Causes

The car's computer may need a hard reset to re-establish communication. Comm2000/Clock Spring Issue: