How trouble codes are structured, the difference between pending, stored and permanent, and why clearing is not fixing.
A DTC — Diagnostic Trouble Code — is how a control unit reports a fault it detected. When a sensor reads outside its expected range, or a plausibility check fails, the unit stores a code and usually lights the dashboard.
Generic OBD-II codes follow a pattern: P0401 — P for powertrain, then four digits. P0xxx is powertrain, B0xxx body, C0xxx chassis, U0xxx network. Manufacturer-specific codes carry more detail and are usually the ones that tell you what actually happened.
Pending codes have been seen once and await confirmation. Stored codes are confirmed. Permanent codes cannot be cleared with a scan tool at all — they clear themselves only after the unit has seen the fault stay away across several drive cycles.
Clearing a code repairs nothing. If the cause is still there the code returns on the next drive cycle, and clearing also resets readiness monitors — which matters at inspection time. Turning a code off inside the software is a different operation, used where the component has been legitimately removed or replaced. Confirming that the change is lawful for the vehicle's intended use is your responsibility.
Published at Sep 4, 2026