GUIDE / OBD-II
Read a code before replacing a part
A diagnostic trouble code records a condition detected by a control module. It does not establish which component caused it. Before clearing anything, save all codes, their status, the reporting module and symptoms. The five-character base code may have extra failure-type digits. Confirm the engine, model year and market before applying a manufacturer's procedure.
Before replacing a part
Record codes and freeze-frame data. Check battery condition, related codes and visible wiring or hose damage. Follow the vehicle's diagnostic procedure and test results before replacing parts.
Scan worksheet →Source-backed code explanations
Sources and interpretation
Read the meaning first, then follow the cited manufacturer pages. The explanations and vehicle-specific examples have different scopes; verify the exact application before using a repair instruction.
- 2000 Powertrain Control/Emissions Diagnosis — Villager OBDII, Description and Operation ↗
Ford Motor Company · PDF 14, 16
Manufacturer example for the 2000 Mercury Villager: module fault detection, code structure and monitor-dependent detection logic. PDF pages are one-based; printed pages 1-14 and 1-16. Historical vehicle-specific documentation, not a current universal code-allocation standard.
- Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and Maintenance Program — EPA420-R-01-015, June 2001 ↗
U.S. Environmental Protection Agency
Historical US technical report, inspection-procedure section: retain diagnostic codes and associated freeze-frame information before repair. Supports preserving evidence; not a current global inspection rule. Text edition has no PDF page anchors.