This sun radiation sensor (sun load sensor) monitors the intensity of incoming sunlight and communicates with the vehicle’s climate control to help maintain stable cabin temperature and comfort. Supplied with protective cover and referenced under part numbers 643604 and 643676, this unit is a direct-fit replacement for affected Citroën and Peugeot models.

Application And Function

The sensor is used by automatic HVAC systems to measure sun load and to adjust blower speed, blend doors and temperature regulation accordingly. Typical fitment includes Citroën C8 and Peugeot 807 models that share the same climate control strategy. Common search terms: sun sensor, sun load sensor, solar radiation sensor, climate control sensor, 643604, 643676.

Installation Recommendations

Location: Usually mounted on the upper dashboard near the base of the windshield, often under a small cosmetic cover. Replacement steps (general guidance):

• Remove the dashboard trim or cover that hides the sensor while taking care not to damage clips.
• Disconnect the electrical connector and unplug the sensor.
• Remove any retaining screws or clips and replace the sensor with the new unit, including the supplied cover.
• Reconnect the connector, reassemble trim and verify proper seating of the cover to avoid light leakage or shading of the sensor.
• After installation, check the automatic climate control operation. If the HVAC control stores fault codes, clear them and verify normal function.

Why This Part Commonly Fails

Sun sensors are exposed to constant UV radiation, heat cycles and temperature extremes behind the windshield. Common failure causes include gradual degradation of the sensor element (photodiode), plastic cover yellowing or cracking, contamination by dust or adhesives, moisture ingress, or connector corrosion. Failures typically present as incorrect cabin temperature regulation, uneven climate behavior under sunny conditions, or HVAC error traces related to the sun sensor.

Technical Information

• Manufacturer: Citroën / Peugeot (Stellantis)
• Model: Citroën C8; Peugeot 807
• Product Codes: 643604, 643676
• Other Numbers: 643604/643676, 643676, 643604

Notes for Fitment: This item is supplied as a direct replacement sun radiation sensor with cover. Ensure the sensor’s mounting position and connector match the vehicle harness before final installation. Proper seating of the cover and correct alignment toward the windshield are important for accurate readings.

This Sunlight Sensor (Solar Radiation Sensor) is a direct-fit replacement designed for automatic climate control systems in various Citroën and Peugeot models. It measures the intensity of solar radiation striking the vehicle and provides that data to the HVAC control module so the system can automatically adjust air distribution, blower speed and air conditioning load for stable cabin comfort. The part is commonly searched by reference numbers such as 643604, 105182352, 105487416 and A66100100, making it easy to find in parts catalogs.

Function And Application

The sensor monitors exterior sunlight intensity and supplies a voltage or digital signal to the climate control unit. The HVAC system uses this input to compensate for direct sun heating on one side of the cabin (for example, sunlight through the windshield) and to modulate compressor duty cycle and fan speed. Proper operation reduces temperature swings, improves passenger comfort and can lower fuel consumption by optimising air‑conditioning operation.

Compatible Models

Typical applications include, but may not be limited to:

• Citroën C4 Picasso
• Citroën C8
• Peugeot 807

Replacement Procedure (Practical Guide)

Intended For Professional Mechanics And Experienced DIYers. General replacement steps:

• Park vehicle on level ground, switch off ignition and remove key. For safety, disconnect the negative battery terminal if you will be working near airbags or significant wiring harnesses.
• Locate The Sensor: The sunlight sensor is usually mounted on the top of the dashboard close to the windshield (center or slightly offset). Remove the dashboard trim or sensor cover as required.
• Unplug Connector: Release the locking tab on the electrical connector and carefully unplug the sensor harness.
• Remove The Sensor: Remove any retaining clips or screws holding the sensor. Extract the sensor from its mounting pocket.
• Install New Sensor: Fit the new sensor into the mounting position, secure clips/screws and reconnect the electrical plug ensuring a firm connection.
• Reassemble And Test: Refit trim components, reconnect battery if disconnected, start the vehicle and set the climate control to AUTO. Verify that the system responds correctly to changes in sunlight (covering/uncovering the sensor should change fan/compressor behaviour).

In Most Cases The Unit Is Plug And Play And Does Not Require Coding. If The Climate System Shows Faults After Replacement, Perform A Diagnostic Scan And Follow The Manufacturer’s Procedure.

Why This Part Fails

• Environmental Degradation: Continuous exposure to UV radiation, heat cycles and humidity can degrade the sensor element or housing.
• Mechanical Damage And Wear: Cracked housing, impact from objects or damaged clips can allow moisture ingress or misalignment.
• Electrical Issues: Corroded or loose connectors, broken wires in the dash harness or intermittent contacts will cause erroneous readings.
• Contamination: Dust, grime or a cloudy lens over the sensor reduces sensitivity and causes incorrect climate control behaviour.

Installation Recommendations

• Handle The Sensor Carefully And Avoid Touching The Sensing Surface Or Lens.
• Ensure Connectors Are Clean And Free Of Corrosion; Use Contact Cleaner If Necessary.
• Always Refit Trim And Clips Securely To Prevent Rattles And Water Ingress.
• After Replacement, Check HVAC Operation In Both Sunlight And Shade To Confirm Correct Response.
• Use Genuine Or High‑Quality Equivalent Parts Using The Correct Reference Numbers For Best Fit And Durability.

Technical Information

• Manufacturer: Stellantis (Citroën / Peugeot Original Equipment)
• Model: Citroën C4 Picasso; Citroën C8; Peugeot 807
• Product Codes: 105182352 B; 105487416 B
• Additional Numbers: 643604; A66100100

Notes For Mechanics And DIYers

This Sensor Is A Common Wear Item In Vehicles Equipped With Automatic Climate Control. When Replacing, Inspect The Surrounding Dash Area And Wiring Harness For Signs Of Moisture Or Damage. Because The Unit Directly Affects Passenger Comfort, Quick Diagnosis And Replacement Restore Correct HVAC Behaviour And Improve Customer Satisfaction.