Engine Control Module: Description and Operation: Overview

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.
Conditions must be met during the diagnostic in order for the diagnostic to be completed. The time and conditions for the diagnostic vary depending on the component or function being diagnosed. Certain diagnostics only require the ignition to be switched on and off for a diagnostic to be run. Other diagnostics require that several different conditions are met. 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 and status are stored in the diagnostic trouble code memory and are visible during normal diagnostic trouble code readout. If the diagnostic trouble code (DTC) lights the malfunction indicator lamp (MIL), this occurs at the same time. If an existing fault disappears and does not recur, the engine control module changes the status of the diagnostic trouble code after a certain number of fault-free operating cycles (depending on the nature of the diagnostic trouble code). This means that it can no longer be read during normal diagnostic trouble code readout. In this status, the diagnostic trouble code remains until the engine control module (ECM) erases diagnostic trouble codes or is disconnected from power.

Substitute value
For certain types of diagnostic trouble codes (DTCs), the missing signal is replaced with a substitute value so that the system can continue functioning.