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.
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