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.

Product Overview

High-quality airbag control unit (SRS/ACU) designed for Citroën C4 (first generation). This unit monitors crash sensors, seatbelt pretensioners and airbag deployment circuits, and issues deployment commands in the event of an impact. Supplied with OEM reference numbers for easy identification and direct replacement in compatible vehicles.

Key Benefits

• Direct fit for many Citroën C4 I models — simplifies replacement and reduces downtime.
• OEM product codes included in the listing for faster search and verification.
• Essential safety component: restores SRS system functionality when faulty or previously deployed unit is replaced.

How It Works

The airbag control unit continuously monitors inputs from impact sensors, occupant sensors and seatbelt buckle sensors. On detecting a severe collision pattern, the unit triggers the appropriate airbags and pretensioners within milliseconds. It also records fault codes and crash events that help diagnose system status after an incident.

Replacement Procedure

Basic replacement steps for experienced mechanics or competent DIYers:

1. Disconnect the negative battery terminal and wait at least 10–15 minutes to discharge residual power in the SRS circuits.
2. Locate the unit (position varies with trim—commonly mounted under the centre console, beneath the passenger seat or behind the glovebox).
3. Remove necessary trim panels and secure fixings to access the control unit.
4. Carefully unplug the electrical connectors; avoid pulling on wiring and protect connector latches.
5. Unbolt the unit, replace with the new unit and reconnect connectors, ensuring a firm fit.
6. Reassemble trim, reconnect the battery and perform a full SRS system scan and fault clear with a compatible diagnostic tool.

Technical Information

• Manufacturer: Stellantis (Citroën)
• Model: Citroën C4 I (First Series)
• Product Codes: 9653493780, 603726100
• Additional Numbers: 654635, 654636

Installation Recommendations

• Always isolate the battery before starting work and follow the vehicle manufacturer’s procedure to prevent accidental airbag deployment.
• Handle the unit and connectors with care to avoid electrostatic discharge and mechanical damage.
• After installation, use a compatible diagnostic tool (for PSA vehicles e.g. DiagBox/Lexia/PP2000) to read and clear SRS fault codes and verify correct operation of the system.
• If the unit has recorded crash data or deployment history, follow legal and safety regulations regarding replacement.

Why This Part Fails

Common causes of airbag control unit failure include: impact/crash events that trigger deployment, water ingress or corrosion damaging internal electronics, connector corrosion or wiring faults, and electrical surges. In many cases the unit remains serviceable unless it has been involved in a collision or exposed to moisture.

Notes For Buyers

Match the OEM part numbers listed above with the vehicle’s original part marking to ensure compatibility. This listing includes primary OEM references used on first-series Citroën C4 models.