Independent diagnostic reference

OBD-II Trouble Codes, Explained Clearly

Understand what the code means, how urgent it is, and what to check before you buy parts.

Use the letter shown by your scanner, followed by four digits.
Meaning in plain EnglishDiagnosis in a sensible orderSafety before repair advice
OBD-II
P0300

Random or multiple cylinder misfire detected

High priorityCheck soon
Start with evidenceRead companion codes and freeze-frame data before replacing parts.

Browse by vehicle system

Find the system behind the code

A DTC points to a circuit or system. Choose the group shown by your scanner to see definitions, symptoms and diagnostic checks.

Engine Misfire Codes diagnostic illustration01

Engine Misfire Codes

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Fuel, Air and Sensor Codes diagnostic illustration02

Fuel, Air and Sensor Codes

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Emissions, EVAP and Catalyst Codes diagnostic illustration03

Emissions, EVAP and Catalyst Codes

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Ignition System Codes diagnostic illustration04

Ignition System Codes

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Transmission Codes diagnostic illustration05

Transmission Codes

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Speed, Idle and Support Codes diagnostic illustration06

Speed, Idle and Support Codes

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Computer and Electrical Codes diagnostic illustration07

Computer and Electrical Codes

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ABS, Brake and Traction Codes diagnostic illustration08

ABS, Brake and Traction Codes

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Airbag and Body Codes diagnostic illustration09

Airbag and Body Codes

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Network and Communication Codes diagnostic illustration10

Network and Communication Codes

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A better way to diagnose

The code is a clue, not a parts list

Good diagnosis follows the evidence. Our guides separate what the control module detected from what may have caused it.

  1. 1

    Read the complete scan

    Record companion codes, status and freeze-frame conditions before clearing anything.

  2. 2

    Check simple causes first

    Inspect connectors, wiring, fluid levels, hoses and recent work before buying a component.

  3. 3

    Test, then repair

    Compare live data and measurements with specifications for the exact vehicle.

Recently published

Start with these diagnostic guides

One carefully structured guide from each diagnostic system, with new pages published daily.

Built for useful answers

Clear enough for drivers. Careful enough for diagnosis.

Generic or manufacturer-specific

We explain when a definition is standardized and when the exact vehicle matters.

Safety before convenience

Brake, airbag, oil-pressure and flashing-light warnings are treated with the urgency they deserve.

No parts cannon advice

Probable causes are not presented as confirmed failures. Testing comes before replacement.

Readable on the driveway

Short sections, ordered checks and mobile-friendly layouts make guides easy to use beside the car.

A practical OBD-II code reference

Car Code Wiki covers powertrain, chassis, body and network diagnostic trouble codes. The library connects short scanner definitions with the questions drivers actually ask: Can I keep driving? What symptoms fit? Which checks should come first? What evidence confirms the repair?

Generic information is useful for orientation, but the exact year, make, model, engine and transmission remain important. Always compare a guide with current service information for the vehicle in front of you.