On-board diagnostic capabilities are incorporated into the hardware and software of a vehicle's on-board computer to monitor virtually every component that can affect emission performance.Checked 2026-08-12
Powertrain Control Module (PCM)
The onboard computer that runs the engine and, on many cars, the transmission.
Also called: PCM · ECM · ECU · engine computer
What it means
Definition
The powertrain control module — also called the ECM or ECU depending on the manufacturer — is the computer that manages engine operation and, on many vehicles, the transmission. CARB describes on-board diagnostic capability as incorporated into the hardware and software of a vehicle's on-board computer to monitor virtually every component that can affect emission performance. When it finds such a fault it stores a code and lights the dashboard lamp, which Missouri's inspection programme describes as a dashboard light that comes on when the vehicle's computer finds a problem that could cause excess pollution.
Why a buyer cares
What it changes
It is the module a scan tool talks to, so its stored codes are the record a buyer can read directly.
In practice
Where you meet it
On repair invoices and scan-tool printouts as PCM, ECM or ECU.
Two things that look alike
Not the same as
The body control module handles windows, locks and lighting rather than engine operation.
Read next
Related terms
OBD-II (On-Board Diagnostics II)
The standardised self-diagnostic system in modern cars, read through a connector under the dash.
Diagnostic Trouble Code (DTC)
The code an OBD-II system stores to identify which monitored system it found a fault in.
Limp mode
The common name for a protective state in which a control module limits engine power after a fault.
Where this comes from
Sources
A dashboard light that comes on when the vehicle's computer finds a problem that could cause excess pollution.Checked 2026-08-12
Naming (PCM vs ECM vs ECU) and whether the module also controls the transmission vary by manufacturer; no citable source was found for the everyday engine-management duties such as fuelling and ignition timing.