OBD-II Diagnostic Trouble Codes
On-Board Diagnostics II reference for light- and medium-duty vehicles. Powertrain (P), body (B), chassis (C), and network (U) fault categories standardized under SAE J2012.
OBD-II is the on-board diagnostics standard required on every gasoline and most diesel passenger and light- to medium-duty commercial vehicle sold in the U.S. since 1996. The codes follow a five-character format defined by SAE J2012 — a letter for the system (P, B, C, U), a digit for generic vs. manufacturer scope, and three hex digits for the specific fault. Use this hub to look up any individual code by family, or jump straight to the most-cited codes from the main Diagnostic Code Reference.
OBD-II Code Ranges
Codes begin with a letter indicating the system and a digit indicating generic (0/2) vs. manufacturer-specific (1/3).
| Prefix | Category | Description |
|---|---|---|
| P0xxx | Generic Powertrain | Engine, transmission, and drivetrain codes defined by SAE. Common in roadside inspections. |
| P1xxx | Manufacturer Powertrain | OEM-specific engine and transmission codes. Interpretation varies by manufacturer. |
| P2xxx | Generic Powertrain (Extended) | Additional SAE-defined powertrain codes introduced in later OBD-II revisions. |
| P3xxx | Generic/Manufacturer Powertrain | Mixed SAE and OEM powertrain codes. |
| B0xxx–B3xxx | Body | Airbags, seatbelts, HVAC, instrument cluster. Less common in CMV inspections. |
| C0xxx–C3xxx | Chassis | ABS, traction control, stability systems. Relevant to brake inspections. |
| U0xxx–U3xxx | Network/Communication | CAN bus, module communication faults. Can indicate wiring or ECU issues. |
Reading OBD-II Codes
OBD-II fault codes follow a 5-character format: a letter identifying the system (P, B, C, U), a digit indicating generic (0, 2) or manufacturer-specific (1, 3) scope, and three hex digits identifying the specific fault. For example, P0300 is a generic powertrain code for random/multiple cylinder misfire detected.
In commercial vehicle inspections under the FMCSA Level I and II protocols, inspectors may connect a scan tool to the vehicle's OBD-II port (typically a 16-pin DLC near the driver's seat) to retrieve active and pending codes. Active emissions-related codes (e.g., EGR, SCR, DPF faults) can contribute to roadside out-of-service decisions depending on the state and inspection level.
OBD-II is standard on light- and medium-duty vehicles from 1996 onward in the U.S. Heavy-duty diesel trucks use the J1939 protocol instead — see the J1939 SPN/FMI reference for heavy-duty diagnostic codes.
Common Powertrain Code Families
- P0100–P0199 — Fuel and air metering (MAF, MAP, O2 sensors)
- P0200–P0299 — Fuel and air metering, injector circuits
- P0300–P0399 — Ignition system and misfires
- P0400–P0499 — Auxiliary emissions (EGR, evap, secondary air)
- P0500–P0599 — Vehicle speed, idle control, auxiliary inputs
- P0600–P0699 — Computer output circuits
- P0700–P0899 — Transmission codes
Featured OBD-II Codes
A starter set of OBD-II diagnostic codes from our reference. Click any code for full meaning, severity, and diagnostic guidance.
More OBD-II codes in our database
Sorted by FMCSA inspection citations. Click any code for the full meaning, severity, and related diagnostic guidance.
Frequently asked questions about OBD-II codes
What does an OBD-II code look like? ▾
Where is the OBD-II port on a truck? ▾
Will an OBD-II code clear itself? ▾
Does an OBD-II code always trigger the check-engine light? ▾
Can OBD-II codes affect emissions inspections? ▾
Related pages
More to explore
Data sources & freshness
TruckCodex aggregates official public-sector datasets. See the Source registry for dataset-level coverage and the Freshness log for last-import timestamps.
Census, SAFER, SMS, Licensing & Insurance (L&I), roadside inspections, crashes, and authority history.
Vehicle recall campaigns, defect investigations, and consumer safety complaints (SCRS).
Cross-border carrier registry and Canadian recall campaigns where applicable.
TruckCodex is an independent aggregator; it is not affiliated with FMCSA, NHTSA, EIA, or Transport Canada. Always verify compliance-critical information directly with the originating agency.