OBD-II glossary

OBD-II

The standardised on-board diagnostics system fitted to every car sold in the US since 1996.

On-Board Diagnostics generation two. It defines the connector, the pin layout, the communication protocols and, critically, a shared list of trouble codes. That standardisation is why the same code number means the same thing on a Honda and a Ford.

Diagnostic Trouble Code (DTC)

The five-character code the engine computer stores when a monitored value falls outside its expected range.

A DTC records that a test failed. It never names the broken part. P0420 says catalyst efficiency measured below threshold; it does not say the converter is bad. Treating a code as a part number is the single most expensive mistake in DIY diagnosis.

Freeze frame

A snapshot of sensor values captured at the exact moment a code set.

When the computer stores a code it also saves engine speed, load, coolant temperature, fuel trim and vehicle speed from that instant. It tells you the conditions the fault appeared under, which is often more useful than the code itself — a misfire at 2,000 RPM under load is a different problem from a misfire at idle.

Fuel trim

The percentage correction the computer applies to the base fuel calculation.

Short term fuel trim is the instantaneous correction; long term is the learned average. Zero means the computer's fuel calculation matches what the oxygen sensor reports. Positive means it is adding fuel because the mixture reads lean. Anything beyond roughly plus or minus ten percent is worth investigating.

Bank 1 / Bank 2

The side of the engine containing cylinder number one is bank 1.

Only V-configuration and flat engines have two banks. On an inline four or six there is only bank 1. Getting this backwards means replacing a perfectly good sensor on the wrong side of the engine.

Upstream / downstream sensor

Upstream sits before the catalytic converter, downstream sits after it.

Also called sensor 1 and sensor 2. Upstream measures what the engine produced and drives fuel trim. Downstream measures what came out of the converter and grades the converter's work. They do completely different jobs despite often being the same part number.

Drive cycle

A specific pattern of driving that lets the computer run its self-tests.

Monitors only run under conditions they were designed for — certain speeds, loads and temperatures. After clearing codes you must complete a drive cycle before the computer can confirm the repair, which is why an emissions test right after a repair often fails on readiness rather than on emissions.

Readiness monitor

A self-test the computer runs on one emissions subsystem.

Each shows as complete or incomplete. Most states will fail an emissions test if too many monitors are incomplete, even with no codes stored at all. Clearing codes resets every monitor to incomplete.

Misfire counter

A per-cylinder tally of combustion events the computer judged to have failed.

The computer detects a misfire from a momentary slowdown in crankshaft speed. Watching which cylinders accumulate counts separates a single-cylinder fault from a condition affecting the whole engine.

Live data

Sensor values streamed in real time from the engine computer.

The single feature that separates a useful scan tool from a code reader. Almost every diagnosis worth doing depends on watching a value change while you make something happen — not on reading the code number.

Bidirectional control

The ability to command a component from the scan tool instead of only reading from it.

Commanding the fuel pump on, cycling the EGR valve, or turning a cooling fan lets you test a component directly. It usually separates mid-range tools from entry-level ones.

Pending code

A fault detected once, not yet confirmed by a second occurrence.

Most emissions codes must fail on two consecutive drive cycles before the computer turns on the check engine light. A pending code is an early warning, and it is the best time to catch an intermittent fault.

Limp mode

A protective state where the computer restricts power to prevent damage.

Often a fixed low RPM limit, locked transmission gear or reduced throttle. It means the computer has decided continuing normally risks serious harm. Diagnose rather than clear and drive on.