The Electronic Control Unit (ECU) is required to maintain the exhaust emissions at acceptable levels. The module is a small, solid state computer which receives signals from many sources and sensors; it uses these data to make judgments about operating conditions and then control output signals to the fuel and emission systems to match the current requirements.
Engines coupled to electronically controlled transaxles employ a Vehicle Control Module (VCM) to oversee both engine and transaxle operation. The integrated functions of engine and transaxle control allow accurate gear selection and improved fuel economy.
In the event of an ECU failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although at a reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is always affected when the ECU enters this mode.