ECU glossary

Spec sheets use the same words for different things. Terms that change meaning by brand carry a Watch out note, so you know what to ask before you buy.

Fitment

Wire-in ECU: A standalone ECU wired to the engine with a new loom, or the factory loom rewired to suit. Fits anything, takes the most work.
Direct plug-in: Replaces the factory ECU in the factory location and plugs straight into the factory loom.
Watch out Some "plug-ins" need extra wires for features the factory loom never had, such as a wideband, flex sensor or drive-by-wire. Check the install notes.
Adapter harnessPlug 'n' Play: A short loom that joins a standalone ECU to the factory ECU connector.
Watch out "Plug and play" sometimes means the harness only, with the ECU sold separately. The finder adds the ECU price so totals compare fairly.
Patch loom / swap kit: An ECU with an engine-specific harness, used for engine swaps (LS, JZ, K-series).

Fuel and ignition

Sequential injection: Each injector fires once per engine cycle, timed to its own cylinder. Needs one injector output per cylinder and a cam signal.
Watch out "Sequential" also describes sequential gearboxes. A unit can support one without the other.
Injector and ignition outputsINJ / IGN: How many injectors and coils the ECU can drive directly.
Watch out Counts aren't always like for like. Some brands count outputs that can be either injectors or general-purpose, and direct injection needs a separate high-voltage driver.
Direct injectionDI / GDI: Fuel is sprayed straight into the cylinder at very high pressure, as in most modern engines. Needs a DI driver, built in or external.
Lambda and widebandλ / WB: Lambda is the air-fuel ratio relative to the ideal (1.00). A wideband oxygen sensor measures it across the whole range.
Watch out "Lambda input" can mean a built-in wideband controller, a CAN-connected controller, or a plain voltage input for an external one. Only the first needs no extra box.
Knock control: Listens to engine vibration through knock sensors and pulls timing when detonation is detected.
Watch out Some ECUs only monitor and log knock. Others retard timing per cylinder automatically. Check which before relying on it.
Flex fuel: Reads ethanol content from a sensor and adjusts the tune to match the blend.
Watch out Basic setups only blend fuel. Fuller setups also blend ignition, boost and cold start tables.

Control features

Traction controlTC: Reduces engine torque when the driven wheels spin faster than they should.
Watch out This one varies the most. Proper systems compare driven and undriven wheel speeds, which needs wheel speed sensors. Simpler systems watch how fast engine rpm rises and guess at wheelspin. Some brands also call boost-by-gear or a fixed torque ramp "traction control". How torque is cut also differs: ignition retard, spark or fuel cut, closing the throttle, or dropping boost. A "yes" on a spec sheet doesn't tell you which you get.
Launch controlTwo-step: Holds the engine at a set rpm on the line so the car leaves with boost built.
Watch out "Two-step" can be a plain second rev limiter, or a full launch system with timing, boost and fuel control and a release strategy.
Anti-lagALS: Keeps the turbo spinning off throttle by retarding timing and letting extra air past the throttle. Hard on turbos and exhausts.
Boost control: Runs the wastegate solenoid to hit a target boost.
Watch out Open-loop runs a fixed duty table. Closed-loop chases a target and corrects itself. Spec sheets often just say "boost control".
Gearbox control: Covers anything from flat-shift ignition cut for sequential gearboxes to full automatic transmission control.
Watch out Running an automatic's solenoids (a 4L60E, say) is a much bigger job than cutting spark on a paddle shift. Some ECUs only talk to a separate transmission controller.
Drive-by-wireDBW / E-throttle: The ECU controls an electronic throttle body instead of a cable.
Watch out Some ECUs need an expansion loom or extra module for drive-by-wire. The number is how many throttle bodies it can run.

Hardware and data

ECU / EMS: Engine control unit, or engine management system: the computer that runs fuel, ignition and everything around them.
VCU: Vehicle control unit. An ECU plus power distribution, logging and wideband in one box.
PDM: Power distribution module. Electronic fuses and relays that the ECU or dash can control.
CAN bus: Controller Area Network, the two-wire network ECUs use to talk to dashes, keypads, modules and factory electronics.
Watch out "CAN support" can mean only that it streams data to a dash, or that it integrates with the factory body, ABS and cluster. Ask which.
Onboard logging: Records sensor data inside the ECU while you drive. Without it you need a laptop plugged in.
Watch out Memory size matters less than how many channels and how fast it records.
Firmware packages: Some ECUs (MoTeC M1, for example) are sold as hardware with the feature set bought separately as a firmware licence. Factor that into the price.