System diagnosis

Another important feature of microcomputer engine control systems is their ability to diagnose failures in their control systems and alert the operator. Sensor and actuator failures or mis-adjustments can be easily detected by the computer. For instance, the computer will detect a malfunctioning MAF sensor if the sensor’s output goes above or below certain specified limits, or fails to change for long periods of time. A prime example is the automatic adjustment system just discussed. If the open-loop calculations consistently come up wrong, the engine control computer may determine that one of the many sensors used in the open-loop calculations has failed.

If the computer detects the loss of a primary control sensor or actuator, it may choose to operate in a different mode until the problem is repaired. The operator is notified of a failure by an indicator on the instrument panel (e.g., check engine). Because of the flexibility of the microcomputer engine control system, additional diagnostic programs might be added to accommodate different engine models that contain more or fewer sensors. Keeping the system totally integrated gives the microcomputer controller access to more sensor inputs so they can be checked.