2017 Peterbilt 579 Bulb Size Guide

Quick Reference Bulb Size Chart: 2017 Peterbilt 579

A note on OEM variations: The 2017 Peterbilt 579 was available with both standard halogen headlight assemblies and upgraded factory-installed LED units. Higher trim levels or optional packages often included the LED headlights and sometimes LED brake/tail lights. It is crucial to identify which system your truck is equipped with before purchasing replacement bulbs, as the fitment and electrical requirements are different.

LED vs. Halogen Upgrade Considerations

Upgrading your 2017 Peterbilt 579's lighting to LED is a popular choice for its brightness, longevity, and modern appearance. However, there are a few key things to keep in mind before you make the switch.

First, for forward lighting like headlights and fog lights, it's not as simple as just plugging in an LED bulb. The original halogen housings are designed to reflect light from a filament in a very specific location. LED bulbs, with their multiple diodes, often don't replicate this perfectly, which can lead to a scattered, ineffective, or even illegal beam pattern that can blind other drivers. For the best and safest results, consider a full housing replacement designed specifically for LED use. Peterbilt offered an excellent OEM LED headlight assembly which is a direct, high-quality replacement.

For other locations like turn signals and brake lights, you may encounter "hyperflash" (the turn signal blinking rapidly) or a "bulb out" warning on your dash. This is because LED bulbs draw significantly less power than incandescent bulbs, and your truck's computer (the Body Control Module) thinks the bulb is burned out. To fix this, you will need to install CANBUS-compatible LED bulbs or wire in an appropriately sized load resistor for each upgraded bulb to mimic the power draw of the original.

How to Replace Each Bulb

Replacing bulbs on the 579 is generally straightforward, requiring basic hand tools.

• Headlights & Front Turn Signals: Access is gained by opening the hood. The headlight assembly is held in by a few easily accessible bolts. Once the assembly is removed, the bulbs can be accessed from the rear by twisting the dust cover off, disconnecting the wiring harness, and turning the bulb to release it. Be careful not to touch the glass of new halogen bulbs with your bare hands, as skin oils can cause them to fail prematurely.

• Fog Lights: These are typically accessed from underneath the front bumper. You may need to remove a small splash shield. The bulb is similar to the headlight, requiring a simple twist to remove and replace.

• Tail Lights & Rear Bulbs: The rear combination lamps (tail, brake, turn, reverse) are housed in a single unit. To access the bulbs, you'll need to remove the screws holding the lens cover in place. The bulbs themselves are a simple push-and-twist or pull-out design.

• License Plate & Side Markers: These small bulbs are usually held in small housings. The license plate light often has a snap-on cover. Side markers might require removing one or two screws to access the bulb from behind.

• Interior Dome/Map Lights: Use a small, flat-head screwdriver or a plastic trim removal tool to gently pry the clear lens off the overhead console. The "festoon" style 578 bulb sits between two metal clips. Be gentle to avoid breaking the plastic tabs.

Common Lighting Issues for this Generation

The 2017 model year of the 579 is known for its reliability, but like any heavy-duty truck, it can experience some common lighting issues. The primary point of failure is often not the bulbs themselves, but the wiring and connectors. Vibrations from the road can cause connections to loosen or wires to chafe, leading to intermittent or complete failure. Check the wiring harnesses leading to the headlight and tail light assemblies for any signs of damage or corrosion, especially around the connectors.

Some owners have reported issues with the daytime running light (DRL) function failing, which is often traced back to a faulty relay or a setting within the Body Control Module that may need to be reset by a dealer or a qualified shop.

Finally, water intrusion into the headlight or tail light housings can cause chronic bulb failure and corrosion. If you notice condensation inside the lens, it’s best to remove the housing, dry it out completely, and inspect the seals for damage before reinstalling.

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