OBD-II & Diagnostic Trouble Code Glossary

Understanding these terms will help you diagnose check engine lights, interpret scan tool data, and communicate effectively with mechanics.

OBD-II

On-Board Diagnostics version 2 — the standardized vehicle diagnostic system required in all US cars since 1996. Provides DTCs via the 16-pin DLC port.

DTC

Diagnostic Trouble Code — an alphanumeric code (e.g., P0420) stored by the ECU when a vehicle system malfunction is detected.

ECU / ECM

Engine Control Unit / Engine Control Module — the computer that manages engine operation, fuel injection, ignition timing, and emissions.

PCM

Powertrain Control Module — combines the ECU and TCM into a single unit that controls both engine and transmission functions.

MIL

Malfunction Indicator Lamp — the 'Check Engine' light on the dashboard that illuminates when the ECU detects a fault and stores a DTC.

Freeze Frame

A snapshot of engine data (RPM, speed, coolant temp, fuel trim) captured at the moment a DTC is set. Helps diagnose intermittent faults.

Pending Code

A DTC that has been detected once but not confirmed. If the fault occurs again on a subsequent drive cycle, it becomes a confirmed code and turns on the MIL.

Drive Cycle

A specific sequence of driving conditions (cold start, highway, idle) required by the OBD-II system to run all emission monitors to completion.

Readiness Monitor

Self-test routines run by the ECU to verify emissions systems (catalyst, EVAP, O2 sensors, etc.) are working correctly.

Fuel Trim

The ECU's adjustment to the air-fuel mixture. Short-term fuel trim (STFT) is instant; long-term fuel trim (LTFT) is a learned adjustment.

Bank 1 / Bank 2

Bank 1 is the engine side containing cylinder #1. Bank 2 is the opposite side. Relevant for V6/V8 engines with dual exhaust.

Upstream / Downstream O2

Upstream (Sensor 1) is before the catalytic converter; downstream (Sensor 2) is after. The ECU compares both to evaluate catalyst efficiency.

Catalytic Converter

An emissions device that converts harmful exhaust gases (CO, HC, NOx) into less harmful substances using platinum-group metal catalysts.

EVAP System

Evaporative Emission Control System — captures fuel vapors from the gas tank and routes them to the engine for combustion instead of releasing to atmosphere.

EGR

Exhaust Gas Recirculation — a system that routes a portion of exhaust back into the intake to reduce NOx emissions and combustion temperatures.

MAF Sensor

Mass Air Flow sensor — measures the amount of air entering the engine. The ECU uses this data to calculate the correct fuel injection amount.

MAP Sensor

Manifold Absolute Pressure sensor — measures intake manifold vacuum/pressure to help the ECU determine engine load and adjust fuel delivery.

CAN Bus

Controller Area Network — the communication protocol used by modern vehicles allowing ECUs, sensors, and modules to share data over a two-wire bus.

Mode 6 Data

Advanced OBD-II test results showing pass/fail thresholds for each readiness monitor. Useful for predicting future failures before a code sets.

Generic vs Manufacturer Code

P0xxx codes are SAE-standard (generic). P1xxx codes are manufacturer-specific. Both follow the same DTC format but P1 codes need brand-specific documentation.