Diagnosis System

DIAGNOSIS SYSTEM

When troubleshooting On-Board Diagnostic (OBD II) vehicles,
the vehicle must be connected to the OBD II scan tool (in compliance
with SAE J1978). Various data output from the vehicle's ECM can then be read.




OBD II regulations require that the vehicle's on-board computer
illuminates the Malfunction Indicator Light (MIL) on the instrument
panel when the computer detects a malfunction in:
1. The emission control system/components
2. The powertrain control components (which affect vehicle emissions)
3. The computer.
In addition, the applicable Diagnostic Trouble Code (DTC) prescribed by SAE J2012 are recorded in the ECM memory. If the malfunction does not reoccur in 3 consecutive trips, the MIL goes off automatically but the DTCs remain recorded in the ECM memory.




To check DTCs, connect the OBD II scan tool to the Data Link Connector 3 (DLC3) of the vehicle. The scan tool also enables you to erase the DTC and check the Freeze Frame Data and various forms of the engine data (Refer to DIAGNOSIS TROUBLE CODE CHART.




NORMAL MODE AND CHECK MODE
The diagnosis system operates in Normal Mode during normal vehicle use. In Normal Mode, "2-trip detection logic" is used to ensure accurate detection of malfunctions. A Check Mode, is also available to technicians as an option. In Check Mode, "1-trip detection logic" is used for simulating malfunction symptoms and increasing the system's ability to detect malfunctions, including intermittent malfunctions.

2-TRIP DETECTION LOGIC
When a malfunction is first detected, the malfunction is temporarily
stored in the ECM memory This is known as 1st trip detection. If the ignition switch is turned OFF and then ON again, and the same malfunction is detected again, the MIL will illuminate. This is known as 2nd trip detection.

FREEZE FRAME DATA
The freeze frame data records the engine conditions (fuel system, calculated load, engine coolant temperature, fuel trim, engine speed, vehicle speed, etc.) when a malfunction is detected. When troubleshooting, freeze frame data can help determine if the vehicle was running or stopped, if the engine was warmed up or not, if the air/fuel ratio was Lean or Rich, and other data from the time the malfunction occurred.