The malfunction indicator lamp or MIL is a warning light on the instrument panel. It is lit up by the control unit, to warn of malfunction that effects engine operation and emission control compliance.
The malfunction indicator lamp can also indicate fault codes. They can be a series of lamp blinks, or voltage pulses from the control unit. They give a number for the kind of fault that is occurring. 1 lamp blink followed by a pause, then 3 blinks in rapid succession indicates code 13, which may mean, for that application, that the oxygen sensor voltage is not changing, leaving the engine in open-loop operation.
When the control unit does not receive a signal from a component, it uses a substitute value from its memory bank. The engine may still operate near its normal level, and the driver may not realize a problem has occurred.
In some cases, if the fault is in a major component, the control unit may go to a “Limp Home” mode, with a fixed injection duration, and ignition setting. The vehicle remains driveable, but with reduced performance, allowing it to be driven to a convenient service point.
With a data scanning tool, an engine management function can be viewed. All of these functions together are a snapshot of the state of all inputs and outputs at that instant. This data can then be analyzed.
A control unit can use data from its memory to adjust engine settings over time. This is called adaptive learning. It only occurs with feedback, or closed loop operation. This vehicle is using medium throttle, with light loading and mild acceleration. Its control unit will set fuel delivery for a specified pulse-width. However, if fuel pressure is less than when the vehicle was new, feedback from the oxygen sensor can cause the lambda control to increase injection time. The control unit learns the new time, and stores it in its memory for future use in that condition.
If the battery is disconnected, or memory is lost, the settings revert to the manufacturer’s programmed specifications, and the vehicle then has to be driven in a range of conditions, to let the control unit, in effect, re-learn the engine’s fuel settings.
