A fault code (DTC) is a 5-character code stored by the car's computer when a sensor reads outside expected limits. P-codes are powertrain (engine/transmission), B-codes body, C-codes chassis, U-codes network. The code tells you which circuit has a fault, not which part — replacing parts based on codes alone is the most common DIY mistake.
Fault codes are useful breadcrumbs but they're not diagnoses. Here's what to actually do with one.
What does P0420 mean? — Catalyst efficiency below threshold (Bank 1). Could be cat, O2 sensor, exhaust leak, or engine running rich. Needs proper diagnosis.
Can I clear codes without fixing the fault? — Yes, but the code returns within 50–100 miles of driving and the EML comes back.
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