2020 GMC Sierra 2500HD — Bulb Sizes (Headlight, Tail, Fog)

As an ASE-certified master technician, I’ve seen my fair share of heavy-duty trucks roll into the bay. When GMC redesigned the Sierra 2500HD for the 2020 model year, they significantly upgraded the lighting systems, making them bolder and brighter. However, finding the correct replacement bulbs for this behemoth can be slightly confusing because your bulb sizes depend heavily on your trim level. The workhorse Pro and SLE trims often feature replaceable halogen bulbs, while the higher-end SLT, AT4, and Denali trims are equipped with fully integrated LED housings.

Below is your comprehensive guide to the 2020 GMC Sierra 2500HD lighting system, designed to help you buy the right bulbs the first time.

Quick reference

Note: This table reflects the replaceable bulbs found on halogen-equipped trims (typically Pro and SLE). If your truck has the premium LED package (SLT, AT4, Denali), the low beams, high beams, and taillights are sealed LED units requiring full assembly replacement if they fail.

LED vs halogen upgrade considerations

If you own a Pro or SLE model with halogen headlights, you might be tempted to upgrade to aftermarket LED bulbs to match the look of the Denali. Before you make the switch, there are a few critical things you need to know:

• CANBUS Errors: Modern GM trucks utilize a highly sensitive CANBUS system that monitors the electrical draw of your bulbs. Because LEDs draw significantly less power than halogens, the truck's computer might think the bulb is burned out. This results in dashboard warning messages, hyper-flashing turn signals, or even bulb rejection (shutting off power to the light). You will likely need LED bulbs with built-in CANBUS decoders or inline load resistors.
• Beam Pattern Adjustments: The Sierra's halogen headlight housings utilize reflectors designed specifically for the focal point of a halogen filament. When installing LED bulbs, ensure they are adjustable. The LED chips must face exactly 3 o'clock and 9 o'clock to mimic the halogen filament and provide a safe, glare-free cutoff line.
• Dust Cover Clearance: Upgraded H11 and 9005 LEDs often feature bulky cooling fans or heat sinks. Ensure the aftermarket bulb you choose will fit behind the factory dust caps to keep moisture out of the housing.

How to replace each bulb

Tackling bulb replacements on a heavy-duty truck often requires a bit of reach. Here are some quick technician tips for the 2020 Sierra 2500HD:

• Low and High Beam Headlights: Accessing the back of the headlight assembly from the top of the engine bay is incredibly tight due to the airbox and battery layouts. The easiest method is to turn the steering wheel inward, remove the handful of Torx screws holding the front of the inner wheel well liner, and peel it back. You'll have direct access to the H11 and 9005 dust caps.
• Fog Lights: These can be accessed from underneath the front bumper. If you have the factory skid plate, you may need to remove a few bolts to gain comfortable access to the PSX24W bulb sockets. A simple quarter-turn counterclockwise removes the bulb.
• Tail, Brake, and Rear Turn Signals: Drop the tailgate and locate the two T15 or T20 Torx screws on the inner edge of the taillight housing. Remove them, and pull the entire housing straight back toward you (do not pry sideways, or you will snap the plastic alignment pins). Twist the sockets counterclockwise to swap your 7444 bulbs.
• Reverse Lights: These are located in the same taillight housing as the brake lights. Follow the same procedure and swap out the 921 bulb.
• License Plate Lights: Reach up behind the rear bumper, grasp the bulb socket, and twist it counterclockwise to pull it out. Pull the old 194 bulb straight out of the socket.

Common lighting issues for this generation

While the 2020 Sierra 2500HD is a robust machine, there are a couple of lighting-specific quirks to watch out for:

1. Moisture in the Housings: Whether it's the halogen or premium LED setups, some Sierra owners have reported condensation building up inside the headlight or taillight lenses. If the condensation forms large droplets or pooling water, the seal is compromised. If your truck is under warranty, GM will typically replace these assemblies.
2. Hyper-flashing Turn Signals After Towing: Occasionally, the trailer lighting module can cause the truck's turn signals to hyper-flash even after a trailer has been disconnected. This usually clears up after a quick ignition cycle, but if it persists, it may require a body control module (BCM) software update.

If your Sierra HD is experiencing persistent electrical gremlins or you need a professional to tackle those hard-to-reach headlight bulbs, don't hesitate to get expert help. Find a trusted shop on Repairius.