2025 Kenworth W900 Bulb Sizes: Complete Guide to Headlight, Tail, and Fog Lights

As an ASE-certified master technician who has spent countless hours tracing electrical lines and swapping out housings on heavy-duty rigs, I know how critical reliable lighting is for safety and compliance. The 2025 Kenworth W900 is an iconic piece of American highway machinery. While it retains the classic long-hood styling that drivers love, its electrical and multiplexing systems have advanced significantly.

Whether your W900 rolled off the assembly line with basic halogen fixtures or premium factory LED upgrades, keeping your truck fully illuminated is non-negotiable. Below is your comprehensive guide to the 2025 Kenworth W900 bulb sizes.

Quick reference

For the 2025 model year, Kenworth offers a mix of traditional replaceable bulbs and completely sealed LED modules depending on your spec sheet.

Note: Many 2025 W900 configurations bypass traditional replaceable bulbs entirely for exterior lighting, utilizing commercial-grade sealed LED assemblies. Always verify your specific housing before purchasing.

LED vs halogen upgrade considerations

If your fleet or owner-operator spec opted for halogens, upgrading to LEDs is one of the smartest investments you can make for nighttime visibility. However, heavy-duty trucks present unique challenges:

• Multiplexing and CANBUS: Modern Kenworth W900s utilize a centralized Cab Electrical Control Unit (CECU). If you replace a standard halogen 4x6 with an aftermarket LED without the proper resistance, the CECU may detect a drop in current and trigger a "Bulb Out" warning on your dash, or even cut power to that circuit. You may need LED anti-flicker harnesses or load resistors.
• Heated LEDs for Winter: Standard halogen bulbs generate heat that melts snow and ice off the headlight lenses. LEDs run cold. If you run northern routes in the winter, strongly consider upgrading to heated 4x6 LED modules (like those from J.W. Speaker or Truck-Lite) to prevent ice build-up from blinding you in a blizzard.
• Sealed Assemblies: Converting a traditional incandescent pedestal turn signal to LED usually requires replacing the entire housing, not just swapping an 1157 bulb, to ensure maximum weatherproofing.

How to replace each bulb

Working on a W900 is generally straightforward thanks to the expansive hood, but a few tips will save you time:

• Headlights (Low/High Beam): The classic dual rectangular headlights are secured by a chrome bezel and a metal retaining ring. Remove the Torx or Phillips screws holding the bezel, then unscrew the inner retaining ring. Do not turn the aiming screws unless you plan to realign the headlights afterward. Unplug the 3-prong (low/dual) or 2-prong (high) pigtail, apply dielectric grease to the new connection, and reinstall.
• Cab/Marker Lights: Most bullet or watermelon lights on the sun visor and cab roof require removing the two Phillips screws on the lens. If it’s a sealed LED, you will need to pull the housing out to access the quick-disconnect wiring harness tucked inside the roof cavity.
• Rear Tail/Brake/Turn Lights: Standard DOT rear lights are usually 4-inch rounds held in place by a rubber grommet. Push the old assembly out from the rear of the bracket, unplug the 3-wire pigtail, plug in the new sealed unit, and pop it back into the grommet.
• Interior Dome Lights: Use a plastic trim tool to pry the clear or opaque covers off the overhead console. Swap the 211-2 festoon or 194 wedge bulbs carefully, as the metal retaining clips can be brittle.

Common lighting issues for this generation

While the 2025 W900 represents the pinnacle of Kenworth's classic line, electrical gremlins can still occur. The most common issue on these trucks isn't bulb failure—it's corrosion at the chassis harness connections. Road salt and magnesium chloride can infiltrate the pigtails at the rear light brackets or the bumper-mounted fog lights. Always use a generous amount of dielectric grease on any exposed connection. Additionally, if an entire bank of lights (e.g., all left-side markers) goes out, check the power distribution box for a tripped auto-resetting breaker or check the CECU programming before assuming the bulbs are bad.

Keeping a heavy-duty truck on the road requires heavy-duty expertise. If you're dealing with persistent wiring faults or multiplexing errors, don't leave it to guesswork. Find a trusted shop on Repairius.