Car Fog light Guide

One of a pair of lights, mounted at the front of a car, below the headlights, generally white or yellow in colour, for increased visibility ahead in foggy driving conditions.


To understand how fog lights work, we have to first take a look at how fog works. Fog is a regular occurrence in many parts of the world, especially around large bodies of water. When the conditions are right, tiny water droplets or ice crystals can easily float in the air, just over the surface of the ground. In addition to acting like a cloud and blocking your view, the tiny water droplets or ice crystals decrease visibility even further through reflecting light that hits them. This makes driving through fog with headlights, and especially high beams, a frustrating as well as dangerous experience.

Fog lights are a solution for this, and take a different approach to all that water vapour hanging over the road. They work in different poor visibility conditions, including fog, mist, snow, and even blowing sand. In order to let you see the road through the fog, factory-equipped fog lights are put as low as reasonably possible, mostly in the lower bumper cover on most modern vehicles. Despite its appearance, fog usually doesn’t touch the ground but hovers about 18 inches over the surface. The low placement of the fog lights on a vehicle lets the light beam hit the road without any interference. With a sharp cut-off angle in the top of the light housing, none of the beam shines up to reflect off the fog and only illuminates the road ahead. Angled downward, fog lights do not have the “reach” of headlights, but they will light up ample road ahead for drivers to slowly make their way to their destination. Since the fog lights aren’t just reflecting light back, other drivers are more aware of your presence, making for a second safety benefit.

Symptoms of a Bad or Failing Fog Light Relay

Common signs have a clicking sound from the relay and the fog lights either not turning on or not switching off.

The fog light relay is the electrical relay that controls power to the fog lights. The fog lights are lights underneath the headlights that are made to improve visibility in dense fog or rain conditions. They are meant to help the driver see the edges of the road whenever the visibility is low. When the fog light relay fails or has any issues, it can result in problems with the operation of the fog lights. Mostly a bad or failing relay will produce a few symptoms that can alert the driver of a potential issue.

1. Clicking sound from the relay

One of the first symptoms of a problem with the fog light relay is an audible clicking noise coming from the relay. Many relays will make a quiet audible click when they are switched on, however, an excessively loud click may be an indicator that the relay might be having internal issues.

2. Fog lights are always on

Fog lights that remain on at all times are yet another symptom of a problem with the fog light relay. If the fog light relay shorts internally or has any wiring issues, it can result in the fog lights to remain on at all times. This is obviously an issue, as it will be draining the battery of the vehicle as well as significantly shorten the life of the bulbs.

3. Fog lights do not turn on

Another common symptom of a faulty fog light relay will be the fog lights that do not come on at all. The fog light relay is the switch that helps to control power to the fog lights, and it is mostly controlled either automatically by the computer or manually by the driver. If the relay fails, it will disable the power to the fog lights and make them inoperable.

How to Replace a Fog Light Relay

Fog lights aid the driver’s visibility when driving in dense fog. Clicking sounds as well as malfunctioning lights are signs of a bad fog light relay.

Most of the vehicles today come equipped with fog lights. Fog lights were originally intended to help with vision when there was fog present. For this reason, most manufacturers will usually mount the fog lights in the front bumper, or on the lower valance.

Symptoms of a failing fog light relay consist of a clicking sound when turned on or improper functioning of the fog lights. The most common location of the fog light relay is in the under hood fuse and in the relay box. The under hood fuse/relay box can be mounted in any of several different places under the hood. It can be mounted on the driver or passenger side, and on the front or rear of the engine compartment.

Part 1 of 1: Replacing a fog light relay

Materials Required

  • Relay puller pliers (optional)
  • Screwdriver set

Step 1: Locate the under hood fuse/relay box. Open the hood and locate the fuse/relay box. Manufacturers will usually label the box with the word “Fuse” or “Relay” on the lid.

Step 2: Remove the under hood fuse/relay box cover. The lid of the fuse/relay box can generally be removed by hand, but sometimes a small screwdriver may be necessary to gently pry the retaining tabs back to release them.

Step 3: Identify the fog light relay to be replaced. Identify the fog light relay that has to be replaced. Most manufacturers will provide a diagram on the lid of the under hood fuse/relay box that will show the place and function of each fuse as well as the relay placed inside the box.

Step 4: Remove the fog light relay to be replaced. Remove the fog light relay that have to be replaced. This can usually be done by pinching it between your fingers and pulling it up and out, or by using a pair of pliers.

It is not uncommon to wiggle it back and forth while pulling on it.

  • Note: You can also utilize a small screwdriver to gently pry a fuse or relay from its position, as long as you are very careful not to make contact with the metal terminals on them. This can cause a short circuit and lead to additional issues.

Step 5: Match the replacement fog light relay to the original. Compare the replacement fog light relay to the one which is being removed. Ensure it has the same basic dimensions, the same amperage rating, and that the terminals are in the same number and orientation.

Step 6: Insert the replacement fog light relay. Line up the replacement fog light relay with the recess out of which the old one came out. Gently set it in place and push down till it stops. The base needs to be flush with the fuse box deck and be approximately the same height as the relays around it.

Step 7: Reinstall the under hood fuse/relay box cover. Place the under hood fuse/relay box cover back to the fuse/relay box and then, push down on it until it engages the retaining tabs. There should be an audible click when it engages or a felt click.

Step 8: Confirm replacement of the fuse relay. Once everything has been reinstalled, just turn the ignition to the “run” position. Turn the fog lights on and verify the fog lights’ operation.

Video of How to Install Fog Lamp

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