OBD-II Fault Code Decoder – Look Up Diagnostic Codes
Enter an OBD-II fault code to find its meaning, possible causes and the vehicle systems that may be affected.
Enter a code, code prefix or term. Examples: P0420, P04 or catalyst.
The lookup loads data from FRALIMO only. Your input is not sent to a diagnostic service, and no personal search history is saved.
OBD code reference: selected fault codes
These codes have dedicated explanations with considerations for further diagnosis.
Understanding OBD-II fault codes
What is an OBD-II fault code?
A diagnostic trouble code (DTC) is an entry recorded by the vehicle’s diagnostic system. It describes a detected condition in a monitored system, helping to narrow down further investigation. A code alone is not evidence that a particular replacement part is needed.
How are OBD fault codes structured?
The format supported here contains five characters, such as P0420. The first letter identifies the vehicle system. The remaining characters identify the detected condition; they can include the letters A to F.
| P | Powertrain: engine and transmission |
|---|---|
| B | Body |
| C | Chassis |
| U | Communication between control modules |
Classification reference: Bosch OBD1150, section 3.6 (PDF).
What does P0420 mean?
P0420 describes catalyst system efficiency below the threshold on bank 1. The measurements used to assess it depend on the vehicle. The code calls for investigation of the system; it does not automatically justify replacing the catalytic converter. Read about P0420 in detail.
Example of monitor operation: Ford OBD-II System Operation Summary 2013, Catalyst Monitor (PDF).
What does an illuminated check engine light mean?
The light indicates that a fault has been detected; stored codes provide more specific clues. A steady light and a flashing light require different consideration. The light alone cannot identify a defective component or establish whether it is safe to continue driving. Follow the owner’s manual.
If a warning light flashes or the vehicle shakes severely or overheats, stop safely and seek qualified assistance. A general online explanation cannot assess the vehicle’s condition.
Background: EPA: OBD Frequently Asked Questions.
Can I read a fault code myself?
A compatible OBD scanner, or an adapter with diagnostic software, can read supported codes. Access to individual control modules depends on the vehicle and the equipment. This decoder explains codes you have already read; it does not connect to your car. Follow the vehicle and scanner instructions.
Do OBD codes mean the same thing for every manufacturer?
No. There are standardised and manufacturer-controlled ranges. P0 generally identifies a generic range; P1 identifies manufacturer-specific definitions. The allocation of other ranges also depends on the code family. Manufacturer-specific entries need documentation for the relevant vehicle.
Does a code identify the defective part directly?
No. An unexpected measurement can have several causes. Other codes, the circumstances, stored operating data and appropriate testing all contribute to a diagnosis. Lists in this decoder are possible starting points, not confirmed diagnoses or repair instructions.
What if my fault code cannot be found?
Check all five characters, including the difference between the digit 0 and the letter O. This database contains generic OBDex entries and does not cover every manufacturer diagnostic system. Keep the scanner’s complete readout and consult relevant manufacturer documentation or a qualified workshop if needed.
Data source and limitations
The lookup uses a static copy of OBDex hosted by FRALIMO. Its generic records do not provide comprehensive manufacturer diagnostics. Additional information has not been checked for every vehicle; missing, conflicting or overly generic information is flagged or omitted. Repair cost and repair time estimates are not displayed.
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