Introduction

INTRODUCTION
The ECM (ECCS control module) has an on board diagnostic system, which detects engine system malfunctions related to sensors or actuators. The ECM also has a memory function which detects various emission related diagnostic information.

This includes:
Diagnostic Trouble Code (DTC) Mode 3 of SAE J1979
Freeze Frame (F.F.) data Mode 2 of SAE J1979
System Readiness Test (SRT) code Mode 1 of SAE J1979
1st trip Diagnostic Trouble Code (1st trip DTC) Mode 7 of SAE J1979
1st trip Freeze Frame (1st trip F.F.) data
Test values and Test limits Mode 6 of SAE J1979




These data can be verified using procedures listed in the image table.

The malfunction indicator lamp (MIL) on the instrument panel lights up when the same malfunction is detected in two consecutive trips (Two trip detection logic), or when the ECM enters fail-safe mode.

ON BOARD DIAGNOSTIC SYSTEM FUNCTION
The on board diagnostic system has the following four functions.




1. BULB CHECK :
This function checks the bulb for damage (blown, open circuit, etc.) of the malfunction indicator lamp.
2. MALFUNCTION WARNING :
This is a usual driving condition. When a malfunction is detected twice in two consecutive trips (2 trip detection logic), the malfunction indicator lamp will light up to inform the driver that a malfunction has been detected.
Only the following malfunctions will light up or blink the MIL even in the 1st trip.
- "Misfire (possible three way catalyst damage)"
- "Closed loop control"
- Fail-safe item (except "START SIGNAL CIRCUIT")
3. SELF-DIAGNOSTIC RESULTS :
By using this function, the DTCs and 1st trip DTCs can be read.
4. FRONT HEATED OXYGEN SENSOR MONITOR :
In this mode, the fuel mixture condition (lean or rich) monitored by front heated oxygen sensor can be read.


TWO-TRIP DETECTION LOGIC
When a malfunction is detected for the first time, 1st trip DTC and 1st trip F.F. data are stored in the ECM memory. The MIL will not light up at this stage (1st trip).
If the same malfunction is detected again during the next drive, the DTC and F.F. data are stored in the ECM memory, and the MIL lights up. The MIL lights up simultaneously when the DTC is stored (2nd trip). The "trip" in the "Two Trip Detection Logic" means performing of the "DTC CONFIRMATION PROCEDURE".




Specific on board diagnostic items will cause the MIL to light up or blink, even in the 1st trip, as shown.

DTC AND 1ST TRIP DTC
The 1st trip DTC (whose number is the same as the DTC number) is displayed for the latest self-diagnostic result obtained after the ECM memory is cleared. When the self-diagnosis results in "NG" for the 1st trip, the 1st trip DTC is stored in the ECM memory. If the self-diagnosis results in "OK" for the 2nd trip, the 1st trip DTC will be cleared from the ECM memory. If, on the other hand, the self-diagnosis results in "NG" for the 2nd trip, both the DTC and the 1st trip DTC will be stored in the ECM memory and the MIL will illuminate. In other words, the DTC is stored in the ECM memory and the MIL illuminates when the self-diagnosis results in "NG" in two consecutive trips. If a non-diagnostic operation (For example: Driving pattern A.) is performed between the 1st and 2nd trips, only the 1st trip DTC will continue to be stored in the ECM memory. For items whose diagnosis results in "NG" after only one trip (the MIL illuminates), both the DTC and the 1st trip DTC will be stored in the ECM memory. The 1st trip DTC is also cleared if diagnosis is not performed, but two driving pattern A are performed after the MIL has illuminated.
The 1st trip DTC, along with the DTC, is cleared from the ECM memory in a method outlined later. For items whose 1st trip DTCs are displayed, refer to database. These items are prescribed by legal regulations to continuously monitored system/components. However, other items also can be displayed on the CONSULT screen or with the ECM set in Diagnostic Test Mode II (Self-diagnostic results).
1st trip DTC detection is performed without causing the MIL to light up. This does not warn the driver of a problem. Also, the result of the 1st trip DTC detection does not bring the vehicle owner any disadvantage when the vehicle is taken in for the I/M test. When the 1st trip DTC is detected, Nissan first clears it and then tries to perform "DTC confirmation procedure" or "Overall function check" to analyze the problem. If the problem is duplicated, Nissan determines the problem as a malfunctioning item, requiring repair.
The 1st trip DTC is specified in Mode 7 of SAE J1979.

FREEZE FRAME DATA AND 1ST TRIP FREEZE FRAME DATA
The ECM has a memory function, which stores the driving condition such as fuel system status, calculated load value, engine coolant temperature, short fuel trim, long fuel trim, engine speed, vehicle speed at the moment the ECM detects a malfunction.
Data which are stored in the ECM memory, along with the 1st trip DTC, are called 1st trip freeze frame data, and these data, stored together with the DTC data, are called freeze frame data. The 1st trip freeze frame data can only be displayed on the CONSULT screen, not on the GST The data can be displayed for only one item. Therefore, the ECM has the following priorities to update the data.




For example, the EGR malfunction (Priority: 2) was detected in two consecutive trips and the freeze frame data was stored. After that when the misfire (Priority: 1) is detected in another trip, the freeze frame data will be updated from the EGR malfunction for the misfire. If data already stored in the ECM memory and data which occurs later have the same priority, the preceding freeze frame data remains unchanged in the ECM memory while the 1st trip freeze frame data is replaced by the most recent data. Both the freeze frame data and 1st trip freeze frame data are cleared from the ECM memory, along with DTC using procedures explained later.

SYSTEM READINESS TEST (SRT) CODE
Inspection/Maintenance (I/M) tests of the on board diagnostic (OBD) II system may become the legal requirements in some states/areas. All SRT codes must be set in this case. Unless all SRT codes are set, conducting the I/M test may no longer be allowed.
SRT codes are set after diagnosis has been performed more than two times. This occurs regardless of whether the diagnosis results in "OK" or "NG", and whether or not the diagnosis is performed in consecutive trips.




The image table lists the four SRT items (11 diagnoses) for the ECCS used in Y32 models.
Together with the DTC, the SRT code is cleared from the ECM memory using the method described later. This means that after ECCS components/system are repaired or after battery terminals remain disconnected for more than 24 hours, all SRT codes are possibly cleared from the ECM memory.