B173d Peugeot

Fault code B173D in Peugeot vehicles indicates a communication failure between the vehicle’s security system and the key. Specifically, it means the car is struggling to identify either the mechanical/electronic key or the hands-free remote control when it is in the reader. Symptoms and Causes

This error typically affects the vehicle's immobilizer system, which may result in:

Starting Issues: The engine may fail to start or the dashboard may display a "key not detected" warning.

Security Lockouts: The immobilizer may remain active, preventing the engine management ECU from unlocking the vehicle for operation.

Root Causes: These often range from a weak key fob battery or a faulty internal transponder to issues with the reading antenna, wiring, or the Built-in Systems Interface (BSI). Recommended Troubleshooting Steps

Before pursuing expensive repairs, follow these common diagnostic steps:

Check the Key Fob: Replace the key fob battery and attempt to use a spare key to rule out a localized transponder failure.

Verify Battery Voltage: Low vehicle battery voltage can cause "ghost" electrical faults in Peugeot's sensitive electronics. Ensure the battery maintains roughly when the engine is off. b173d peugeot

Perform a BSI Reset: Many electronic communication errors can be cleared by performing a "soft reboot" of the BSI module. This involves a specific sequence of disconnecting and reconnecting the battery.

Inspect the BPGA/Battery Protection Unit: On newer models like the 308 or 3008, a fault in the battery protection and management unit can mimic key recognition errors.

In the context of Peugeot and Citroën (PSA Group) vehicles, the fault code B173D is a diagnostic trouble code (DTC) that signifies a failure in identifying the mechanical or electronic key, or the remote control. Understanding Fault Code B173D

This code is categorized as a "Body" code, typically managed by the BSI (Built-in Systems Interface), which is the central control unit for the vehicle's electronic functions. When this fault is active, the vehicle's immobilizer system fails to recognize the authorized signal from the key fob, leading to several operational issues. Primary Symptoms:

Engine Non-Start: The engine may fail to crank or start because the immobilizer remains active.

Key Not Detected: A warning message such as "Key not detected" or "Electronic anti-theft faulty" may appear on the dashboard.

Intermittent Starting: The car might start occasionally but fail at other times, often during specific weather conditions or after the vehicle has been sitting. Fault code B173D in Peugeot vehicles indicates a

Central Locking Issues: Difficulties locking or unlocking the vehicle remotely. Common Causes

The failure to identify the key can stem from several localized or systemic issues:

Key Fob Battery: A weak or dead battery in the remote control is the most frequent cause.

Key Reader/Antenna: The internal antenna or the key reader unit (often located near the ignition or in the center console for hands-free systems) may be faulty or disconnected.

Electrical Interference: Proximity to other electronic devices or low vehicle battery voltage can disrupt the signal.

Wiring/BSI Communication: Corroded wiring or a software glitch in the BSI can prevent the key's ID from being correctly processed. Recommended Steps for Resolution

Replace the Key Fob Battery: This is the simplest and cheapest first step. Mirror Fails to Fold: When you lock the

Try a Spare Key: If the car starts with a second key, the issue is isolated to the original key fob.

Check Vehicle Battery Voltage: Low voltage in the main car battery can cause "ghost" faults in the BSI. Ensure the battery is fully charged (typically above 12.7V when off).

Perform a BSI Reset: This "soft reset" of the car's computer can sometimes clear temporary communication glitches. A standard procedure involves disconnecting the battery following a specific sequence (e.g., waiting for the system to go to "sleep" before disconnecting).

Professional Diagnostic: If the above steps fail, use a dedicated diagnostic tool (like Diagbox) to check for deeper BSI or wiring faults.

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2. Common Symptoms

When this code is present, you may notice one or more of the following issues:

  • Mirror Fails to Fold: When you lock the car or press the mirror fold button, one or both mirrors do not retract.
  • Mirror Fails to Unfold: When you unlock the car or start the engine, the mirrors remain tucked in or do not extend fully.
  • Clicking Noise: You might hear a repeated clicking sound coming from the door panel or the mirror housing as the system attempts to move the mirror but fails.
  • Warning Message: The dashboard may display a "Mirrors Fault" or "Check Mirror" message.
  • Battery Drain: In rare cases, if the module keeps trying to engage the motor repeatedly due to the fault, it can lead to a small parasitic drain on the battery.

The Fix: How to Clear B173D Permanently

Once you have diagnosed the faulty component, here is the repair path.

Repair / Fix Options

  • No quick reset – Clearing the code without fixing the cause will make it return immediately after ignition cycle.
  • Seat mat replacement – Most common fix. Requires new OEM or high-quality aftermarket occupancy sensor mat (cost: ~€80–150 for part, plus installation).
  • Calibration required after replacement – New mat must be “taught” zero point via diagnostic tool. Without it, fault persists.
  • Repair wiring – If connector or wires damaged, repair with proper repair kit (Do NOT use simple crimps – airbag system requires specific low-resistance splices).
  • Software update – For some models (e.g., early 2014–2015 Peugeot 308 T9), a dealer software update resolves false B173D triggering.