2021 Peterbilt 579 Bulb Size Guide

Quick reference

The 2021 Peterbilt 579, a modern workhorse designed for efficiency and driver comfort, primarily utilizes advanced LED lighting for its critical exterior functions. This often means that key lighting components like low beams, high beams, DRLs, brake lights, and tail lights are integrated LED units from the factory. This design offers superior light output, durability, and energy efficiency compared to traditional halogen bulbs.

However, some auxiliary lighting, such as fog lights, front and rear turn signals, reverse lights, license plate lights, and side markers, commonly still employ replaceable incandescent bulbs. It's important to note that while LED is standard for many crucial exterior lights, specific trim levels or optional packages might introduce variations. For instance, some higher trims may offer full LED packages for all exterior lighting, or aftermarket upgrades might have been installed. Always double-check your specific truck's configuration before purchasing replacement bulbs.

LED vs Halogen Upgrade Considerations

Upgrading from halogen to LED bulbs in a 2021 Peterbilt 579, where incandescent bulbs are still used, can offer numerous benefits, including brighter light, a whiter light color, increased lifespan, and lower power consumption. However, this isn't always a straightforward swap, especially in a modern truck like the 579, which features sophisticated electrical systems.

CANBUS Errors

Many modern vehicles, including commercial trucks, use a Controller Area Network (CANBUS) system to monitor various electrical components, including lighting. When you replace a halogen bulb with an LED, the LED's lower power draw can be misinterpreted by the CANBUS system as a burned-out bulb. This can trigger a "bulb out" warning light on your dashboard or even cause hyper-flashing for turn signals. To prevent this, you'll often need to install CANBUS-compatible LED bulbs or inline resistors (also known as load resistors) to simulate the electrical load of the original halogen bulb. It's crucial to choose quality resistors that are adequately rated to dissipate heat, as they can get very hot.

Adapters Needed

While some LED bulbs are designed to be a direct plug-and-play replacement for their halogen counterparts, others may require specific adapters or connectors to fit properly into the bulb housing and connect to the vehicle's wiring harness. Even though the bulb size designation might be the same (e.g., 3157), the physical dimensions of LED bulbs can sometimes differ, necessitating careful selection and potentially minor modifications.

Beam Pattern Concerns

When upgrading fog lights from halogen to LED, it's essential to consider the beam pattern. Halogen fog light housings are designed to work optimally with the light emission pattern of a halogen bulb. Installing an LED bulb with a different light distribution can result in a scattered, ineffective, or even glaring beam pattern, reducing visibility rather than improving it. Look for LED bulbs specifically designed to mimic the filament placement of the original halogen bulb for the best results.

How to Replace Each Bulb

Fog Light (H10): Typically accessible from underneath the front bumper or by removing the inner fender liner. Unplug the connector, twist the bulb counter-clockwise to remove, and replace. Be cautious of hot bulbs if recently used.

Front Turn Signal (3157/3157LL): Often accessed by reaching behind the headlight assembly or, in some cases, by removing screws to access the lens. Twist the bulb socket counter-clockwise to remove, pull out the old bulb, and push in the new one.

Rear Turn Signal (3157/3157LL): Usually found by removing the lens cover for the taillight assembly. Twist the socket, remove the old bulb, and insert the new one.

Reverse Light (3157/3157LL): Similar to the rear turn signals, access is typically through the taillight lens. Twist the socket and replace the bulb.

License Plate (194): These small bulbs are usually accessed by removing a couple of screws from the clear lens cover above the license plate. Pop out the old bulb and push in the new one.

Side Marker (194): Depending on the location (fender, cab, etc.), these are generally straightforward. Often, you can pry off the lens or twist the bulb socket from the back.

Dome/Map Light (921): Carefully pry off the translucent lens cover for the interior light fixture. The bulb can usually be pulled straight out from its socket. Use a plastic pry tool to avoid marring interior surfaces.

Note for integrated LED units (Low Beam, High Beam, DRL, Brake Light, Tail Light): If one of these integrated LED units fails, it often requires replacing the entire housing or assembly, as the LEDs are not designed for individual replacement. Consult your Peterbilt dealer or a qualified technician for these repairs.

Common Lighting Issues for this Generation

While the 2021 Peterbilt 579 benefits from robust LED lighting, some common issues can still arise, particularly with the incandescent bulbs or related electrical systems:

• Flickering Incandescent Bulbs: This can often be an early sign of a bulb going out, but it can also indicate a poor electrical connection or corrosion in the bulb socket. Check connections and clean any corrosion.
• 'Bulb Out' Warnings (with incandescent bulbs): If a traditional incandescent bulb burns out, the CANBUS system will likely trigger a dashboard warning. Replace the faulty bulb promptly.
• Corrosion in Sockets: Especially prevalent in exterior lights exposed to the elements, corrosion can lead to intermittent operation or complete failure. Regular inspection and cleaning of bulb sockets can prevent this.
• Trailer Lighting Issues: Many lighting problems on a Peterbilt 579 can originate from trailer wiring or connections. Always check the trailer lights and connections when diagnosing issues, especially with turn signals or brake lights.
• DRL Module Failures: While rare, issues with the DRL control module can sometimes lead to intermittent or non-functional daytime running lights. Diagnosis often requires specialized tools.

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