System Description

PURPOSE
The Engine Control Module (ECM) contains a built-in self diagnosis system by which troubles with the engine signal network are detected, a Data Trouble Code (DTC) is stored, and a Malfunction Indicator Lamp (MIL) on the combination meter lights up. By analyzing various signals the ECM detects system malfunctions relating to the sensors of actuators. The self-diagnosis system has two modes, a normal mode and a test mode.

OPERATION
The On-Board Diagnostic system operates as follows:

Two Trip Detection
Certain DTC's can be triggered by driving conditions without a malfunction present. To improve self diagnostic reliability, "Two Trip" detection logic is used.
When a fault is detected, the ECM retains a temporary record of the fault in memory. An actual code is set only if the same fault is recorded during the next ignition / driving cycle. The MIL is only illuminated after the second detection. Because the ECM must see the fault for 2 consecutive trips, the chances of a false code are greatly reduced.

Normal Mode
In normal mode, the self diagnostic system monitors 14 items, indicated by code number. If a malfunction is detected when in the normal mode, the ECM lights up the MIL to inform the driver of the occurrence of a malfunction. The light goes off automatically when the malfunction has been repaired, but the diagnostic trouble code(s) remains stored in the ECM memory (except code no. 16). The ECM stores the code(s) until it is cleared from memory. The diagnostic trouble code(s) can be read by the number of blinks of the MIL as described later. When 2 or more codes are indicated, the lowest number (code) will appear first.

Test Mode
In normal mode, the self diagnostic system monitors 9 items, indicated by code number. A test mode function has been added to the functions of the self-diagnostic system of the normal mode. This function defeats the two trip detection logic and narrows the parameters of acceptable tolerance for the purpose of detecting malfunctions such as poor contacts, which are difficult to detect in the normal mode. When in the test mode, if a malfunction is detected in any of the systems indicated by code numbers 13, 22, 24, 25, 26, 27, 31, 41 or 71, the ECM lights up the MIL to inform the technician of the occurrence of a malfunction. As in the normal mode, the malfunction code is stored in the ECM memory (except codes 43 and 51), even if the ignition is switched OFF, until it is cleared from memory.