For the 2001 GMC Sierra 2500HD, the low-beam headlight uses a 9006 bulb and the high beam uses a 9005 bulb. The supplied fitment data lists one headlight setup and does not identify a separate projector or non-projector variation. If equipped with fog lights, use an 880 bulb; the daytime running light is listed as 4114.
For exterior lighting, the front and rear turn signals use 3157A bulbs. Parking lights also use 3157A, while the tail and brake lights use 3157R. The reverse lights use 3157, the center high-mounted stop light uses 921, and the license plate lights use 194. Front and rear side markers are listed as 194NA.
Interior fitment includes a 211-2 dome bulb, 194 glove-box bulb, 912 trunk bulb, and 93 engine-compartment bulb. Courtesy, instrument, and map-light applications have multiple listed options, so compare the original bulb before ordering: courtesy uses 194 or 2721; instrument uses 161, 194, or 2721; and map lights use 168 or 578.