What is the rear fog light assembly of a car?
The rear fog light assembly is a safety warning device installed at the rear of the vehicle to enhance the visibility of other vehicles in low-visibility severe weather conditions (such as heavy fog, rainstorms, and heavy snow). It is a mandatory legal lighting component that must be installed by law.
Core Components
The rear fog light assembly typically consists of the following four main parts:
Lens: Made of red polycarbonate (PC) material, with a light transmittance of ≥80%, and has anti-glare patterns on the surface.
Reflector: Mostly a parabolic aluminum structure, used to focus light.
Lamp bulb: Commonly an H21W halogen lamp (21W), and some models have upgraded to LED sources.
Electrical interface: Controlled by the vehicle body control module through an independent circuit and relay (usually located in the left compartment of the driver's cabin), which is switched on and off by the vehicle body control module.
Main Functions and Features
Red light source: Wavelength 620–750nm, with strong penetration in fog.
High brightness: Light intensity of 150–300 candelas, much higher than that of ordinary tail lights.
Independent operation: Must be separated from the brake light and marker light circuits to avoid signal confusion.
Installation Position Requirements (according to "Technical Conditions for Motor Vehicle Operation Safety") :
Ground height: 35cm to 1m
Usually located below the rear bumper of the vehicle
Can be single-sided (common in European vehicles) or double-sided configuration
Applicable Scenarios and Regulations
Activation Condition: Extreme weather with visibility lower than 50 meters.
Regulatory Basis: The national standard GB 4785 clearly stipulates the technical requirements such as color, brightness, and installation angle.
Non-decorative Purpose: It is an active safety feature, and its use is related to whether the annual inspection is qualified. It cannot be removed or replaced with non-compliant aftermarket parts at will.
Replacement and Maintenance Tips
Most models require the removal of the rear bumper or wheel arch for replacement.
Some models (such as BYD S6) support not removing the bumper and directly replacing the through the space behind the wheel arch.
After the replacement, it is necessary to test the waterproof sealing performance and the lighting function.
The failure of the rear fog light assembly usually manifests as the rear fog lights not lighting up, an alarm on the dashboard indicating a fault, or in some models, the rear fog lights being abnormally constantly on. According to the current (May 2026) authoritative public information available, the causes of the failure and the troubleshooting methods are as follows:
Common causes of failure
Broken bulb: The filament burns out, the glass becomes black or cracked, which is the most common direct cause.
Fuse blown: The rear fog light circuit usually uses a 10A–15A fuse. A blown fuse will cut off the power supply.
Line problems: Including loose plugs, oxidation, open circuits, short circuits or aging of the lines.
Fog light switch failure: The internal contacts are worn out or have poor contact, resulting in the inability to issue an opening command.
Control module or relay failure: Some models are controlled by the vehicle body control module (BCM) or relays. The failure of the module will cause system abnormalities.
Physical damage to the lamp cover or assembly: Such as collision, water ingress or aging leading to a failure in sealing, affecting the internal circuit.
Recommended troubleshooting steps (in order of priority)
Check the bulb
Turn off the power supply, open the trunk, remove the rear fog light lamp cover.
Observe whether the bulb is blackened, cracked or the filament is broken.
When replacing, do not touch the glass with your hands, as grease may shorten its lifespan; choose a bulb that matches the model (such as the common P21W or W21/5W).
Check the fuse
Find the fuse box (commonly located in the engine compartment or below the driver's seat on the left).
Compare the fuse cover markings to find the corresponding fuse for the rear fog lights.
If the middle metal wire is blown out, replace it with a new fuse of the same specification; if it repeatedly blows out, it needs to be checked for short circuits.
Check the lines and plugs
Unplug the rear fog light plug and check for looseness, corrosion or bent pins.
Use a multimeter to measure the continuity of the lines to confirm there is no open circuit or short circuit.
Test the fog light switch
Observe the dashboard indicator lights when operating the switch.
If there is no response, the internal contacts of the switch may be damaged, and professional repair or replacement is required.
Read the fault code (if possible)
Use an OBD diagnostic tool to read the fault code in the vehicle body control module, which can precisely locate whether it is a sensor, module or communication problem.
Check the control module or relay
If the above are all normal, it may be a BCM or relay failure. Professional equipment is required for detection, and it is recommended to take it to a repair shop for handling.
Safety reminder
The rear fog light is a legal safety device. It must work properly in low visibility conditions. If there is a failure, it should be repaired in time.
If you lack circuit knowledge or tools, do not disassemble the lines yourself. You should go to a professional repair facility for handling.
If you need to further determine the circuit diagram or bulb model of the rear fog light of a specific vehicle model, you can provide the vehicle brand and year for precise query.
If you want to know more, keep reading the other articles on this site!
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