SECURITY, DESCRIPTION AND OPERATION, Description. Diagnostic Self-Test The ECM performs a self test routine every time the ignition is switched to position II. The ECM constantly monitors the system while the ignition is in this position. Any faults found are stored in a nonvolatile fault memory. The fault memory contents can be read using TestBook/T4 (See Diagnostics below. TestBook/T4 also allows for the testing of certain functions and the activation of actuators. When a fault is detected, the component in question is tagged as defective. The tag is initially regarded as provisional and is not changed to a confirmed status until a certain period of time has elapsed. When the fault is changed to confirmed, the ECM adopts the default value for that function, if one is available. Once the fault is classed as confirmed, a fault memory entry is generated. This entry is accompanied by the current mileage and the environmental conditions at the time of the fault occurrence. The fault entry is stored in the ECM EEPROM. If the fault is engine management system related, the ECM may illuminate the MIL in the instrument pack to notify the driver of the fault condition.
ENGINE MANAGEMENT SYSTEM – TD6 DESCRIPTION AND OPERATION 18-1-13