DTC
Workshop glossary › Diagnostics & protocols · UpdatedA five-character code an ECU stores when it detects a fault. Format and meaning are standardised by SAE J2012 (e.g. P0301 = misfire on cylinder 1).
A Diagnostic Trouble Code (DTC) is a five-character identifier the ECU stores when one of its self-tests fails. The format is standardised by SAE J2012: the first character is the system letter, the second is generic (0) or manufacturer-specific (1), and the remaining three are the fault number.
- P – Powertrain (engine, transmission)
- B – Body (airbags, climate, seats)
- C – Chassis (ABS, steering, suspension)
- U – Network/communication (CAN-bus errors)
A code is stored as pending on the first failure, confirmed after the same failure on a second drive cycle, and permanent for emissions-related faults that cannot be cleared by simply pulling the connector — only the ECU's own self-clear logic will remove them. That last category exists specifically to stop drivers from clearing codes before an inspection.
For workshop use, a DTC is a starting point, not a diagnosis. P0420 (catalyst efficiency below threshold) can be a failed cat, a bad oxygen sensor, a vacuum leak, or fuel trim drift — the code only tells you the test that failed, not the part to replace.