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.
Axle Ratio:
- GT4 = 3.42
- GU6 = 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 = Gov-Lok limited slip — quick test: lock the front hubs, put in 4WD, see if the rear locks up under slight throttle
A/C:
- C60 = Front A/C
- C69 = Rear A/C (Suburban)
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.