General 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.

Emission-related diagnostic functions
The engine control module (ECM) checks that the emission-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 memory
When the engine control module (ECM) detects a fault, a diagnostic trouble code with qualifier and status is stored in the control unit's diagnostic trouble code memory. Fault code storage has four status levels: Temporary, Not yet stored, Stored and Erased. When the engine control module (ECM) detects a fault, the diagnostic trouble code is stored with the status Temporary. If the fault persists the next time diagnostics is performed, diagnostic trouble code status switches to Not yet stored. At that time, the diagnostic trouble code will appear during normal DTC readout in VIDA. If the fault is still active during the next operating cycle, the diagnostic trouble code receives the status Stored. If the diagnostic trouble code causes the malfunction indicator lamp (MIL) to illuminate, that process starts now. For more information, see "Lighting the malfunction indicator lamp (MIL)" below.
If an existing fault disappears and does not return, the engine control module (ECM) stores the code (after a number of fault-free operating cycles depending on the fault code and previous status) with the status Erased. In this status, the diagnostic trouble code remains until the engine control module (ECM) is cleared of diagnostic trouble codes or the power to it is cut off.

Hint: A diagnostic trouble code with the status Erased can be read in VIDA using extended DTC readout. This is often useful when explaining a customer-reported symptom that occurred some time ago.

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.

Lighting the malfunction indicator lamp (MIL)
Emission-related diagnostic trouble codes light the malfunction indicator lamp. The lamp is lit when the diagnostic trouble code's status is Stored.