Your OBS truck’s VIN and RPO codes tell you exactly how it left the factory — engine, transmission, axle ratio, options, and more. Here’s how to read them.
Decoding Your VIN (17-Digit)
Using a 1994 Chevrolet C1500 as example: 1GCEC14Z1RE123456
| Position | Digits | Meaning | Example |
|---|---|---|---|
| 1 | 1 | Country (1 = USA) | 1 |
| 2 | G | Make (G = GM) | G |
| 3 | C | Division (C = Chevrolet / K = GMC) | C |
| 4 | E | GVWR Class | E = 4,001–5,000 lbs |
| 5 | C | Carline (C = Conventional / R = Suburban) | C |
| 6 | 1 | Series (1 = 1500 / 2 = 2500 / 3 = 3500) | 1 |
| 7 | 4 | Body Style (4 = Regular Cab / 3 = Ext. Cab) | 4 |
| 8 | Z | Engine Code (see below) | Z = 5.7L TBI |
| 9 | 1 | Check Digit | — |
| 10 | R | Model Year (see below) | R = 1994 |
| 11 | E | Assembly Plant | E = Pontiac, MI |
| 12–17 | 123456 | Production Sequence | — |
VIN Position 8 – Engine Codes (GMT400)
| VIN Code | Engine | Notes |
|---|---|---|
| Z | 5.7L TBI V8 (350) | Most common gas engine |
| H | 5.7L MFI V8 | Multi-port injection |
| W | 4.3L V6 (TBI) | Base engine |
| N | 7.4L V8 (454 BBC) | Big block option |
| F | 6.5L Detroit Diesel | Naturally aspirated |
| S | 6.5L Detroit Diesel Turbo | Turbocharged |
| P | 6.2L Detroit Diesel | Pre-1993 option |
| C | 5.0L V8 (305) | Early GMT400, 1988–1992 |
VIN Position 10 – Model Year Codes
| Code | Year | Code | Year |
|---|---|---|---|
| J | 1988 | R | 1994 |
| K | 1989 | S | 1995 |
| L | 1990 | T | 1996 |
| M | 1991 | V | 1997 |
| N | 1992 | W | 1998 |
| P | 1993 |
Reading Your RPO Codes (Glove Box Sticker)
The RPO sticker is in your glove box — a white label with rows of 3-letter codes. Each code is a factory option your truck was built with.
Rather than working the list by hand, photograph the sticker and run it through the GM RPO Decoder, or check a single code in the GM RPO Codes Database. The codes below cover the common ones; the database covers the rest.
Axle Ratio:
- GU6 = 3.42
- GT4 = 3.73
- GT5 = 4.10
Transmission:
- MT1 = 4L80E automatic
- MXO = 4L60E automatic
- MX5 = 5-speed manual (NV3500)
Transfer Case:
- NP1 = NP231 (part-time 4WD)
- NP3 = NP241 (part-time, HD)
Trailering:
- Z82 = Trailering Special Package
Differential:
- G80 = the rear differential option. On a GMT400 this is the Eaton governor-actuated automatic locker — the one everybody calls the Gov-Lock. It behaves as a locker, not a clutch-type posi, regardless of what a given year’s paperwork calls it. GM’s own documents are genuinely inconsistent on the wording: their 1990 document says “locking rear differential” and their 1991 one says “P, AXLE POSITRACTION, LIMITED SLIP.” Both are in the database as GM-sourced entries.
A/C:
- C60 = Front A/C
- C69 = Rear A/C (Suburban)
Quick Test for a G80
Get both rear wheels off the ground with the truck in neutral, then spin one rear wheel by hand. On an open differential the other wheel spins the opposite direction. On a working G80 the two tend to lock together once you exceed the slip threshold.
Worth knowing: the Gov-Lock’s flyweight mechanism wears out. A high-mileage GMT400 can carry the G80 code on its sticker and still behave like an open diff, so the sticker tells you what it left the factory with, not what it does today.
Where to Find Your Sticker
Open the glove box and look for a white label on the inside panel or back wall. If it’s worn or gone, a GM dealer can sometimes pull options with the last 8 of your VIN, or use the NHTSA public VIN lookup for basic specs.
Post your VIN position 8 and 10 below and I’ll help decode your engine and year. Or paste your RPO codes if you’re trying to figure out what options you have.