By Pablora Parts LLC parts specialists · Updated September 8, 2026 · Part of Vehicle lookup
Every GM vehicle from the mid-1980s onward left the factory with a Service Parts Identification label, usually in the glovebox, sometimes in the trunk or under the spare tire cover. It lists every Regular Production Option the vehicle was built with, in three-character codes. For parts buying it is more useful than the VIN, because it names the exact axle ratio, cooling package and suspension the truck has.
Where to find it and how to read it
Open the glovebox and look at the inside of the door or the back wall. The label shows the VIN, the paint code, and then rows of three-character codes in alphabetical order. Photograph it once and keep the photo on your phone; every parts quote for that truck is faster with it.
The codes that change parts fitment
These are the ones we ask about most often on Silverado, Sierra, Tahoe, Yukon, Suburban and the mid-size trucks.
- Engine: LM7 is the 1999–2007 5.3 V8, LR4 the 4.8, LQ4 the 6.0, LC9 and LMG the 2007–2013 5.3, L83 the 2014–2018 5.3 and L86 the 6.2, LV3 the 4.3 V6. The engine code settles injectors, coils, converters, water pumps and alternators.
- Axle ratio: GU6 is 3.42, GT4 is 3.73, GT5 is 4.10. G80 is the locking rear differential. Wrong ratio means the wrong differential, driveshaft speeds and sometimes the wrong transfer case programming.
- Cooling: V08 heavy-duty cooling, KNP auxiliary transmission cooler, KC4 engine oil cooler. These decide which radiator fits.
- Suspension and drive: Z71 off-road package, Z85 increased-capacity suspension, NHT max trailering, NP1 two-speed transfer case, NP8 Autotrac automatic four-wheel drive. They decide shocks, springs, sway bars and the transfer case model.
- Paint: the WA code, like WA8555 for black, is the color code for any body part you want pre-painted or color-matched.
Why the VIN alone is not enough on a GM truck
The VIN encodes the engine and the body series but not the axle, the cooling package or the suspension. Two 2011 Silverado 1500 crew cabs with the same VIN pattern can have different radiators, differentials and shocks. The RPO label is the only document that separates them, which is why our specialists ask for it before quoting radiators, driveline parts and suspension.
Photograph the label and call. A specialist reads the codes and quotes the exact radiator, differential or suspension part your truck was built with.
Questions we get about this
Yes, with more questions. A GM dealer can print the build sheet from the VIN, and the old part's fittings and markings answer most of what the label would. Photos of the old part help.
Not in the same form. Ford lists axle and transmission codes on the door-jamb certification label; Ram uses a build sheet by VIN; Toyota lists options on the window sticker only. GM's label is the most complete.
Yes. Sierra and Silverado, Yukon and Tahoe, Canyon and Colorado use the same codes and the same label format.