GMC Sierra 3500 Fog Lamp Relay Replacement Cost

Many vehicles today come factory equipped with fog lamps. These front mounted lights are placed near to the ground in the front bumper cover and are used when the vehicle is driven in foggy conditions where visibility is a problem. Light reflecting off of the fog significantly decreases visibility during driving. Because fog tends to be less dense near the ground, the fog lamps illuminate the road better than the headlights that are mounted higher up on the vehicle. The fog lamp relay is used to switch the 12 volt power to the lamps on and off in response to a signal from the fog lamp switch mounted on the dashboard or the steering column. The relay is most commonly located in a fuse block under the hood.

A relay is an electromagnetic device that is operated by a low voltage signal from a switch or electronic control module. When the relay senses the low voltage signal from the control circuit, it switches a higher voltage power circuit on or off. The fog lamps, like many other vehicle components, require a 12 volt power supply to operate. To save cost and weight, the manufacturer tasks a relay with the job of turning the fog lamps (or the headlamps, cooling fan, etc.) on and off. The fog lamp switch sends a lower voltage signal to the relay. The relay, sensing the signal, switches the power to the lamps on or off. This arrangement also allows an Electronic Control Module (ECM) to oversee lighting operations and control various functions.

A relay normally emits a slight clicking sound under the hood when it is engaging, but a bad relay might produce a louder than normal clicking sound without turning the lamps on or off. If the fog lamp relay is failing it might operate intermittently. The fog lamps will work sometimes, but not all the time. Or the fog lamps might not work at all. They stay on, or they stay off. Some of these symptoms can also be caused by worn out bulbs, a broken wire, a blown fuse, a faulty switch, or computer problem.

Many vehicles do not come equipped with fog lamps. It is reasonable to drive without them. However, if the fog lamps are not working properly, visibility while driving in foggy conditions will be reduced. Also, if the relay has an internal short circuit, the fog lamps may not be able to turn off and could drain the battery.

A fog lamp relay is not a maintenance item on a vehicle and does not have a scheduled replacement interval. It should last the lifetime of the vehicle. In some cases, however, it will wear out and need to be replaced.

To diagnose a bad fog lamp relay, a technician will first rule out other possible causes of intermittent or inoperative fog lamps. Simple causes, such as a blown fuse, are checked first. The bulbs in the fog lamps will wear out and need replacement eventually, so the technician will check those. If the fuse and the bulbs are good, the technician will pull the relay from its mounting location and use a multimeter to check the electrical connections for voltage. If voltage is present, the technician will check the resistance across the terminals of the relay and might test the relay by applying power to it. In some cases, the problem might originate in the ECM. In these cases, the technician will look for a diagnostic trouble code (a fault code) that can be read with a scan tool.

Replacement of the fog lamp relay is simple in most cases. It is usually located in a fuse block under the hood. With the hood and the fuse block opened, the technician locates the relay, pulls it out, and installs a new relay.

It is recommended that a qualified technician at a trusted repair shop diagnose an issue with faulty fog lamps. Problems can stem from a failing switch, broken wire, loose or corroded ground, dirty or incomplete connection, worn bulbs, or even ECM malfunction, in addition to a bad relay.

Care must be exercised whenever working on the electrical system of a car, truck, or SUV. Replacement of a fog lamp relay is simple, but basic safety measures should not be overlooked when diagnosing a faulty fog lamp circuit.

The fog lamp relay can be replaced on most vehicles by a person with little DIY experience. The procedure to remove and replace a relay is not very different than that to replace a fuse. However, diagnosis of a lighting system malfunction - whether in the fog lamps or in the headlamps - can be a job that is beyond even someone with more intermediate experience and may require special diagnostic tools, especially if an Electronic Control Module needs to be accessed.