Emergency program
The control module continuously monitors input and output signals.
If any signal is missing or is outside its permitted range the control module connects an emergency program for that signal. The aim is that the car should be driveable even if a fault has occurred.
In the emergency program there are instructions to use another signal to assist certain calculations or to use a fixed value instead of the missing signal.
The number of emergency programs for each system varies as does the number of replacement values which are used in the different cases. However the one signal that no fuel injection or ignition system can do without is the engine speed (RPM) signal. If this is missing the engine stops immediately.
On many systems the function with the emergency program is reversible. This means that if the fault disappears (a loose connection for example) the control module returns to normal function. What is required for this varies between systems. In a number of systems it occurs the next time the ignition is switched on, other systems require the car to be driven a number of times with different loadings for a certain period of time.