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) or the hand-held tester. 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 Lamp (MIL) on the instrument panel when the computer detects a malfunction in: 1) the emission control system/components, or 2) the powertrain control components (which affect vehicle emissions), or 3) the computer. In addition, the applicable Diagnostic Trouble Codes (DTCs) prescribed by SAE J2012 are recorded in the ECM memory (see DIAGNOSTIC TROUBLE CODE CHART ).

If the malfunction does not reoccur in 3 consecutive trips, the MIL turns off automatically but the DTCs remain recorded in the ECM memory.

Fig 1: Identifying Malfunction Indicator Lamp

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

• To check DTCs, connect the hand-held tester or OBD II scan tool to the Data Link Connector 3 (DLC3) of the vehicle. The hand-held tester or OBD II scan tool also enables you to erase the DTC and check the freeze frame data and various forms of engine data (see the instruction manual for the OBD II scan tool or the hand-held tester). The DTC includes SAE controlled codes and manufacturer controlled codes. SAE controlled codes must be set according to the SAE, while manufacturer controlled codes can be set by a manufacturer with certain restrictions (see the DIAGNOSTIC TROUBLE CODE CHART ).
• 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 (hand-held tester only) (see CHECK MODE PROCEDURE ).
Fig 2: Connecting DLC3 To Hand-held tester
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
• *2 trip detection logic:

  When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory (1st trip). If the ignition switch is turned OFF and then ON again, and the same malfunction is detected again, the MIL will illuminate (2nd trip).

• 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.

Priorities for troubleshooting:

When multiple DTCs occur, find out the order in which the DTCs should be inspected by checking the component's DTC chart. If no instructions are written in the DTC chart, check DTCs in the following order of priority:

1. DTCs other than fuel trim malfunction DTCs (P0171 and P0172) and misfire DTCs (P0300 to P0304).
2. Fuel trim malfunction DTCs (P0171 and P0172).
3. Misfire DTCs (P0300 to P0304).

The vehicle's ECM uses the ISO 9141-2 for communication protocol. The terminal arrangement of the DLC3 complies with SAE J1962 and matches the ISO 9141-2 format.

Fig 3: Identifying DLC3 Terminals

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

HINT:

Connect the cable of the OBD II scan tool or the hand-held tester to the DLC3, turn the ignition switch ON and attempt to use the OBD II scan tool or the hand-held tester. If the screen displays UNABLE TO CONNECT TO VEHICLE, a problem exists in the vehicle side or the tester side.

• If the communication is normal when the tool is connected to another vehicle, inspect the DLC3 on the original vehicle.
• If the communication is still impossible when the tool is connected to another vehicle, the problem is probably in the tool itself. Consult the Service Department listed in the tool's instruction manual.

Standard: 11 to 14 V 

If the voltage is below 11 V, recharge the battery before proceeding.

1. The MIL turns on when the ignition switch is turned ON and the engine is not running.

   HINT:

   If the MIL does not turn on, troubleshoot the MIL circuit (see MIL CIRCUIT ).

2. When the engine is started, the MIL should turn off. If the lamp remains on, the diagnosis system has detected a malfunction or abnormality in the system.