OBD-II Code Library

An OBD-II (On-Board Diagnostics II) code, or diagnostic trouble code (DTC), is a standardized five-character code your vehicle’s computer stores when it detects a fault. The first letter shows the system (P, B, C, or U) and the remaining digits narrow it down to a specific subsystem and fault. Reading the code with a scanner is the starting point for diagnosis.

Type the exact code from your scanner into the search box at the top of this page to filter the library, or browse by system. Each guide explains the meaning, symptoms, likely causes, how to diagnose it, and repair costs. If your code isn’t listed, it may be manufacturer-specific — describe your symptoms to the free AI Diagnostic.

The second character tells you: a 0 means a generic SAE code that means the same thing on every make, while a 1 or 2 means the code is defined by the specific automaker and can vary between brands. Most common codes you’ll search — like P0300 or P0420 — are generic. Manufacturer-specific codes often need a make-specific reference to interpret accurately.

Disconnecting the battery can clear stored codes and the Check Engine Light, but it doesn’t fix the underlying problem — if the fault is still present, the code will come back. It also erases other learned settings and readiness monitors, which can cause you to fail an emissions test until the car re-learns them. It’s better to diagnose and repair the cause, then clear the code with a scanner.

Start with the codes that look like a root cause — misfires, fuel/air, and sensor-circuit codes — before secondary codes like catalyst efficiency. A single underlying problem (a vacuum leak, a failing sensor) often sets several codes at once, so fixing the root cause can clear the rest. When in doubt, the AI Diagnostic can help you prioritize.