What Are RPO Codes And Where Do You Find Them?
From 1978 through 1988, General Motors built some of the most iconic mid-size cars on the legendary G-Body platform, including the Chevrolet Monte Carlo SS, Buick Grand National, Oldsmobile 442, and Pontiac Grand Prix. For collectors and restorers, the key to unlocking a G-Body’s true identity is understanding its RPO codes (Regular Production Option codes). These three-character factory codes detail everything from engines and transmissions to axle ratios, suspension packages, paint colors, and trim options. By decoding the RPO label which is usually found in the trunk or glove box, you can verify originality, confirm performance equipment, and trace exactly how your G-Body rolled off the assembly line.
Key G-Body RPOs (1978–1988)
Engines RPO Codes (select G-body–relevant)
LC3 — 3.8L Buick V6 (naturally aspirated)
LC2 — 3.8L Turbocharged Buick V6 (Grand National / GNX / T-Type)
LD5 — 3.8L Oldsmobile 231 V6
LE9 — 5.0L Chevrolet 305 V8 (common across Cutlass, Monte Carlo, Grand Prix)
LG4 — 5.0L Chevrolet 305 V8 (4-barrel carburetor, mid-1980s)
L69 — 5.0L Chevrolet 305 HO V8 (Monte Carlo SS, 442 in later years)
LV2 — 5.0L Oldsmobile 307 V8 (very common late G-body V8)
LM1 — 5.7L Chevrolet 350 V8 (rare G-body option, early years only)
LF9 — 5.7L Oldsmobile 350 Diesel V8
LT1 — 5.7L Chevrolet 350 V8 (fleet/police packages)
Transmission RPO Codes
M15 — 3-speed manual transmission
MM4 — 4-speed manual (rare on G-bodies by the 1980s, but available early)
MD8 — 3-speed automatic (THM200 / THM350 depending on application)
MX0 — 4-speed automatic overdrive (THM200-4R; common in mid/late ’80s G-bodies, including Monte SS, 442, Grand National)
M38 — THM350 automatic (select earlier G-bodies)
Axles / Differentials (ratios & posi) RPO Codes
G80 — Limited Slip Differential (Positraction)
GM8 — 2.56 axle ratio
GU2 — 2.73 axle ratio
GU4 — 3.08 axle ratio
GU5 — 3.23 axle ratio
GU6 — 3.42 axle ratio
GT4 — 3.73 axle ratio (rare, performance models and special orders)
GT5 — 4.10 axle ratio (very rare, fleet/police/race prep)
Suspensions / Chassis RPO Codes
FE1 — Soft Ride suspension (base models)
FE2 — Ride & Handling suspension (improved shocks/springs)
FE3 — Sport suspension (stiffer springs, sway bars; found on 442, SS, GN, etc.)
F41 — Special Performance Suspension (heavy-duty package, often paired with performance engines)
N10 — Dual exhaust system
N41 — Power steering
N67 — Styled aluminum wheels (varied designs by division)
Appearance & Special Package RPO Codes (examples)
D80 — Rear decklid spoiler (Monte Carlo SS, some Grand Prix/442 models)
B84 — Body side moldings
B93 — Additional exterior moldings
TT5 — Halogen headlamps
WU2 — Monte Carlo SS Aerocoupe package (1986–1987)
WE2 — Buick Grand National package
Y56 — Monte Carlo SS Sport package (earlier years)
W42 — Oldsmobile 442 package
Comfort / Convenience RPO Codes (common across brands)
A31 — Power windows
A90 — Power trunk release
AU3 — Power door locks
C49 — Rear window defogger
C60 — Air conditioning
K34 — Electronic cruise control
U75 — Power antenna
UQ1 — AM/FM stereo with clock
UM6 — AM/FM stereo with cassette
UU6 — AM/FM stereo with 8-track player (earlier years)
Safety & Miscellaneous RPO Codes
JA1 — Heavy-duty brake package
JL8 — 4-wheel disc brakes (rare, special performance/police)
N33 — Tilt steering column
K05 — Engine block heater
AU0 — Keyless entry (limited availability in later years)
How to decode your car in 10 minutes
Find the SPID label (trunk/glove box). Snap a clear photo.
Pull the factory option list for your year/model from the GM Heritage Vehicle Information Kit. General Motors
Match codes on your SPID to the Heritage list. For quick cross-checks, use enthusiast RPO dictionaries (MonteCarloSS.com, GBodyForum) but verify anything critical against the Heritage sheet.
Axle/gear confirmation: read your G-code (GM8/GU4/GU5/GU6). If needed, verify mechanically by counting driveshaft rotations or reading the axle tag.
Build sheet & invoices: If you’re missing the SPID, sometimes the build sheet can be found in-car (seat springs, carpet). For concours-level documentation, order records or consult marque clubs; there are even vendors who reproduce SPID labels when you supply VIN and documentation.
Why “all RPO codes” is slippery on G-bodies
Divisional differences: Buick/Olds/Chevy/Pontiac often used different option names for similar content.
Year-to-year changes: Availability and code meanings could shift (e.g., suspension calibrations, axle ratios).
Package stacking: Certain packages automatically forced or excluded other RPOs (noted in price/option sheets). Example: some suspension codes couldn’t be combined with Grand National/WE2 content.
Handy references (bookmark these)
GM Heritage Vehicle Information Kits — official, model/year specific PDFs. General Motors
Monte Carlo SS RPO master lists & decoding guides — great for Chevy G-bodies (cross-check specifics). montecarloss.com+1
GBodyForum RPO threads — community-compiled code lookups for G-bodies. gbodyforum.com+1
Axle ratio decoder (GU4/GU5/GU6/GM8, etc.) — quick reference. crawlpedia.com
Buick Grand National option lists — useful for LC2/WE2 specifics and suspension/axle pairings. jgme01.angelfire.com
GM G-Body RPO Code Reference (1978–1988)
| Category | RPO Code | Description | Models / Notes |
|---|---|---|---|
| Engines | LC3 | 3.8L Buick V6 (NA) | Base Regal, LeSabre-based G-bodies |
| LC2 | 3.8L Turbocharged Buick V6 | Regal T-Type, Grand National, GNX | |
| LD5 | 3.8L Oldsmobile 231 V6 | Cutlass, base models | |
| LE9 | 5.0L Chevrolet 305 V8 | Monte Carlo, Grand Prix, Cutlass | |
| LG4 | 5.0L Chevrolet 305 V8 4-bbl | Mid-1980s Chevy G-bodies | |
| L69 | 5.0L Chevrolet 305 HO V8 | Monte Carlo SS, Olds 442 | |
| LV2 | 5.0L Oldsmobile 307 V8 | Common late ’80s V8 in Cutlass/Regal | |
| LM1 | 5.7L Chevrolet 350 V8 | Rare early G-body option | |
| LF9 | 5.7L Oldsmobile 350 Diesel V8 | Optional diesel engine | |
| LT1 | 5.7L Chevrolet 350 V8 | Fleet/police package option | |
| Transmissions | M15 | 3-speed manual | Rare by the 1980s |
| MM4 | 4-speed manual | Early production, uncommon | |
| MD8 | 3-speed automatic (THM200/350) | Most common early auto | |
| MX0 | 4-speed automatic overdrive (200-4R) | Widespread mid-’80s automatic | |
| M38 | THM350 automatic | Select earlier G-bodies | |
| Rear Axle / Differential | G80 | Limited Slip Differential (Positraction) | Performance & winter packages |
| GM8 | 2.56 axle ratio | Economy gearing | |
| GU2 | 2.73 axle ratio | Common highway gear | |
| GU4 | 3.08 axle ratio | Balanced ratio | |
| GU5 | 3.23 axle ratio | Sporty gearing | |
| GU6 | 3.42 axle ratio | Grand National, Monte Carlo SS | |
| GT4 | 3.73 axle ratio | Performance/police/fleet | |
| GT5 | 4.10 axle ratio | Very rare, special orders | |
| Suspension / Chassis | FE1 | Soft Ride suspension | Base comfort setup |
| FE2 | Ride & Handling suspension | Sportier ride | |
| FE3 | Sport suspension | 442, SS, GN | |
| F41 | Special Performance Suspension | Heavy-duty option | |
| N10 | Dual exhaust system | Performance models | |
| N41 | Power steering | Nearly universal | |
| N67 | Aluminum wheels | Varied by division | |
| Appearance / Packages | D80 | Rear spoiler | Monte Carlo SS, 442 |
| B84 | Body side moldings | Optional | |
| B93 | Extra exterior moldings | Optional | |
| TT5 | Halogen headlamps | Premium lighting | |
| WU2 | Monte Carlo SS Aerocoupe | 1986–1987 | |
| WE2 | Grand National package | Buick Regal | |
| Y56 | Monte Carlo SS Sport | Early years | |
| W42 | 442 package | Oldsmobile Cutlass | |
| Comfort / Convenience | A31 | Power windows | Luxury trims |
| A90 | Power trunk release | Select models | |
| AU3 | Power door locks | Optional | |
| C49 | Rear window defogger | Optional | |
| C60 | Air conditioning | Widely available | |
| K34 | Electronic cruise control | Popular option | |
| U75 | Power antenna | Premium trims | |
| UQ1 | AM/FM stereo w/clock | Base upgrade | |
| UM6 | AM/FM stereo w/cassette | Mid-1980s | |
| UU6 | AM/FM stereo w/8-track | Early years | |
| Safety / Misc. | JA1 | Heavy-duty brakes | Police/fleet |
| JL8 | 4-wheel disc brakes | Rare, performance | |
| N33 | Tilt steering column | Common on higher trims | |
| K05 | Engine block heater | Cold-weather states | |
| AU0 | Keyless entry | Rare late ’80s option |
Decoding the RPO codes on your G-Body is a bit like opening a history book. Whether or not you’re a ‘purist’ it’s still fun to see exactly how your G-Body rolled off the assembly line. If you need any parts for your G-Body, hop on SS396.com or give our friendly techs a call at (203) 235-1200!
