OBD-II Basics
The DLC, the standard PIDs and what the connector can and cannot tell you.
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What you will learn
- Locate and describe the OBD-II diagnostic link connector
- Name the standard diagnostic services (modes)
- Explain generic vs manufacturer-specific codes
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OBD-II standardised the diagnostic connector, the communication protocols and a set of generic trouble codes and live parameters. It gives you a common starting point, but it is not a substitute for manufacturer data — especially for body, chassis and advanced powertrain systems.
The connector provides power, ground and one or more communication pins. Modern vehicles add multiple buses behind a gateway module, so not every system is reachable through a generic tool.
| Code prefix | System | Example |
|---|---|---|
| P | Powertrain | P0301 misfire cylinder 1 |
| B | Body | B1234 (manufacturer specific) |
| C | Chassis | C0035 wheel speed sensor |
| U | Network | U0100 lost communication with ECM |
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Key points
- Standardised since the mid-1990s
- Generic codes start with P0, B0, C0, U0
- Readiness monitors show which tests have run
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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