How to read an OBD-II trouble code
Published · Reviewed and updated
Short answer: A common OBD-II trouble code such as P0420 has five characters identifying a fault category and condition. Record the exact code, module and any additional suffix, then use service information for the vehicle to interpret it.
Finding the port
The diagnostic connector is commonly a 16-pin socket under the driver-side dashboard, sometimes behind a cover. Check the owner manual for its location. Connect and operate the tool according to its instructions and the vehicle procedure.
What the characters tell you
In the common five-character format, P identifies powertrain, B body, C chassis and U network communication. For P0420, the definition concerns catalyst-system efficiency on bank 1. Code ranges include standardized and manufacturer-specific allocations; do not decode an unfamiliar number solely from a simplified digit chart.
Generic does not mean identical
A generic fault category can apply across brands while the hardware, enabling conditions and diagnostic tests differ. Use the complete vehicle-specific definition, related codes, repair history and test results rather than assuming the most expensive named component failed.
Save the record before clearing anything
Record confirmed and pending codes, the reporting module, available freeze-frame values and readiness status. Clearing codes can remove diagnostic evidence and reset readiness. Permanent emissions codes, where supported, follow their own clearing rules and cannot simply be erased with a scanner.
Use the code to choose a test
Check the relevant service procedure and applicable bulletins. Inspect the indicated system, verify measurements and isolate the cause before replacing a part. After repair, repeat the failed test and confirm the required monitor completes successfully.