Automotive Diagnostic Glossary
Diagnostic language shouldn't be a barrier to understanding your vehicle. These are the terms technicians, scan tools, and service data actually use — defined in plain English with examples, related trouble codes, and the tools that work with them.
OBD-II
On-Board Diagnostics, second generation — the standardized vehicle self-diagnostic system required on US light-duty vehicles since 1996.
DTC (Diagnostic Trouble Code)
The alphanumeric code a vehicle stores when its on-board diagnostics detect a fault — like P0300 for a misfire.
VIN (Vehicle Identification Number)
The 17-character unique identifier stamped into every vehicle — the key to exact vehicle data and recall checks.
ECU (Engine/Electronic Control Unit)
The computer that controls engine operation — reading sensors and commanding injectors, ignition, and more.
PCM (Powertrain Control Module)
The computer controlling engine and transmission together — common terminology on many domestic vehicles.
EVAP System
The sealed system that captures gasoline vapor from the tank and stores it for later burning instead of venting it to the atmosphere.
Fuel Trim
The percentage of fuel the computer adds or removes to keep the mixture right — the single most useful number in driveability diagnosis.
Closed Loop
The operating mode where the computer constantly adjusts fuel using oxygen-sensor feedback — normal running for a warmed engine.
Open Loop
The mode where the computer meters fuel from pre-programmed values without oxygen-sensor feedback — normal during warm-up.
Misfire
A combustion event that fails or happens late — the cause of P0300-series codes and a flashing check-engine light.
Catalytic Converter
The exhaust device that converts pollutants into less harmful gases — and the component behind P0420/P0430 efficiency codes.
Oxygen (O2) Sensor
The exhaust sensor measuring oxygen content — the computer's feedback for precise fuel control and converter monitoring.
MAF Sensor
The sensor measuring intake airflow — the primary input for fuel calculation on most modern engines.
MAP Sensor
The sensor measuring intake manifold pressure — used for load calculation and fueling, especially on turbo and speed-density systems.
Throttle Position Sensor (TPS)
The sensor telling the computer how far the driver has opened the throttle — a key input for fueling, timing, and shifting.
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