This airbag control unit (SRS ECU) is a direct-fit module designed for the first-generation Citroën C4. It is suitable for professional workshops and competent DIY mechanics who service SRS systems. The unit restores or replaces the vehicle’s airbag system control, resolving persistent airbag warning lamps or failures caused by prior deployment, electrical faults or physical damage. Common reference numbers for quick searches are included below, improving findability for technicians searching by part number.

Technical Information

• Manufacturer: Stellantis (Citroën)
• Model: Citroën C4 I (First Series)
• Product Codes: 9662643980, 603726100
• Additional Numbers: 654635, 654636

Function and Application

The airbag control unit (also referred to as SRS control module or airbag ECU) continuously monitors crash sensors, seatbelt pretensioners and airbag squibs. In the event of a detected collision, the unit triggers deployment of safety restraints. It also stores fault codes and crash event data used by diagnostic tools. This unit is intended for use in Citroën C4 first-series vehicles; always confirm the part numbers against the vehicle’s repair data when fitting.

Replacement Procedure (Overview)

• Park vehicle on level ground and remove key from ignition.
• Disconnect the negative battery terminal and wait a minimum of 10 minutes to allow system capacitors to discharge.
• Access the SRS control module location (typically inside the passenger compartment; exact placement varies by equipment level and production date).
• Carefully disconnect SRS connectors—do not pull on wiring, release locking tabs first.
• Unbolt and remove the old unit, install the replacement, tighten fasteners to manufacturer torque, and reconnect connectors securely.
• After installation, clear SRS fault codes and perform an SRS system check with an appropriate diagnostic tool to ensure the airbag warning lamp is extinguished and no stored faults remain.

Installation Recommendations

• Observe all SRS safety precautions: wear appropriate PPE, avoid static discharge and never probe airbag connectors with an energized circuit.
• Always disconnect the battery before working on the SRS system and follow the vehicle manufacturer’s removal/installation sequence to prevent damage.
• Use a diagnostic scanner capable of reading SRS fault memory and performing system initialization after replacement.
• If the vehicle has been in a collision, inspect associated wiring, crash sensors and pretensioners; replace any components that show signs of damage or corrosion.
• If in doubt, have the final checks performed by a qualified technician with SRS experience and diagnostic equipment.

Most Common Failure Reasons

• Collision deployment events—modules often become nonfunctional after airbag deployment.
• Water ingress or corrosion affecting connectors and circuit board traces.
• Electrical faults from short circuits, improper jump-starting or incorrect handling during battery disconnection.
• Physical damage from impacts or improper prior repairs.
• Stored crash data or persistent fault codes preventing normal operation and illuminating the airbag warning light.

Notes for Buyers

Part numbers commonly used to locate this unit: 9662643980, 603726100 (additional references 654635, 654636). This description reflects the item’s intended application for Citroën C4 first-series vehicles and is aimed at mechanics and experienced DIYers searching by part number. The unit is described as fully functional in the original listing.