Computers and Control Systems: Description and Operation: Overview

Overview

General
The engine control module (ECM) diagnoses both internal signals and functions as well as signals and functions from connected components.

Conditions for diagnosis
For the diagnosis of a component or function to start, certain specific conditions must be met. The conditions for diagnostics vary depending on the component or function being diagnosed.
The "operating cycle" for the diagnostic must have been run before the diagnostic can be completed. An operating cycle varies depending on the component or function being diagnosed. Certain diagnostics only require the ignition to be switched on and off for an operating cycle to be run. Other diagnostic require that several different conditions are met regarding for example:
- vehicle speed
- engine coolant temperature (ECT)
- time since start
- different load and engine speed relationships in the same trip
- a certain event (for example the evaporative emission system (EVAP) valve is operating).
When the engine control module (ECM) has run all the implemented diagnostics, the control module has completed a "trip". An extensive driving schedule in various conditions is required to complete a trip. The engine may also need to be shut off for a certain amount of time and then switched on again to complete a trip.

Emissions related diagnostic functions
The engine control module (ECM) checks that the emissions related systems are working. These systems are checked by running a diagnostic function. The diagnostic function checks the components and that the system is functioning.

Diagnostic trouble code (DTC) memory
When the engine control module (ECM) detects a fault, the diagnostic trouble code (DTC) is stored with a qualifier and status in the DTC memory in the control module. For certain types of diagnostic trouble codes (DTCs), the missing signal is replaced with a substitute value so that the system can continue functioning.
If the fault disappears, the diagnostic trouble code (DTC) will remain in the DTC memory for some time, but its status will change.

Lighting the malfunction indicator lamp (MIL)
In the event of emissions related diagnostic trouble codes (DTCs), a counter counts down to determine when to light the malfunction indicator lamp (MIL). The conditions for lighting the malfunction indicator lamp (MIL) vary depending on which diagnostic trouble code (DTC) is stored.