What Is DTC (Diagnostic Trouble Code)?
A diagnostic trouble code (DTC) is the standardized alphanumeric code a vehicle's computer stores when it detects a fault. The structure is defined by SAE standards: a letter identifying the system (P for powertrain, B for body, C for chassis, U for network), followed by four digits.
In plain English
A DTC is a starting point, not a verdict. The code tells the technician which condition the computer observed — a misfire, a lean mixture, a converter efficiency reading — but not which part failed. P0171, for example, says the engine runs lean; the cause could be a vacuum leak, a weak fuel pump, or a bad sensor.
Good diagnostic practice pairs the code with the vehicle and its data: freeze-frame conditions, fuel trims, related codes, and live sensor readings. Two vehicles with the same code can have entirely different causes.
Example
Related trouble codes
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