Diagnostics
Check Engine Light Diagnosis: A DIY Process That Avoids Parts Cannon
An OBD-II code tells you which system detected a problem; it does not automatically identify the failed part.
Safety: Work on a cool, secured vehicle when applicable. Follow the service information for your exact vehicle and use proper protective equipment.
Read all codes
Record stored, pending, and permanent codes plus freeze-frame data before clearing anything.
Use the code as a direction
Research the code's definition for the exact vehicle and engine, then inspect wiring, hoses, connectors, and recent repair areas.
Test the suspected system
Use live data, a multimeter, pressure gauge, smoke tester, or other appropriate test instead of replacing the component named in the code description.
Verify the repair
After repair, confirm the symptom is gone and that the relevant monitors and data behave normally.
Bottom line
Diagnose the cause before ordering parts. Vehicle configurations vary, so verify specifications and procedures for your exact year, engine, and model.