SYSTEM OVERVIEW
The ECM monitors engine operation and contains a self-diagnostic system which stores Diagnostic Trouble Codes (DTCs) and complies with On-Board Diagnostics Generation-II (OBD-II) standards. The goal of OBD-II regulation is to provide vehicle with an on-board diagnostic system which is capable of continuously monitoring the efficiency of emission control system, and to improve diagnosis and repair when system failures occur.
The Federal Test Procedure (FTP) sets maximum allowable emission standards. A Malfunction Indicator Light (MIL) must illuminate if a system or component either fails or deteriorates to a point where the vehicle emissions could rise above 1 1/2 times FTP standards.
If malfunction does not reoccur in 3 ignition cycles, the MIL goes off, but DTCs remain recorded in ECM memory. DTCs may only be retrieved using an On-Board Diagnostic (OBD-II) scan tool that complies with SAE standard J-1978 or Lexus scan tool connected to Data Link Connector (DLC) No. 3. The scan tool also provides freeze-frame data and can be used to clear codes.
Two-Trip Detection Logic
Two-trip detection logic is used to prevent erroneous malfunction detection. When a malfunction initially occurs, Diagnostic Trouble Code (DTC) is temporarily stored in ECM memory, but MIL on instrument panel will not illuminate.
The second time that a malfunction is detected, MIL on instrument panel will illuminate, provided ignition is turned off and then back on after first malfunction was detected. This is referred to as the 2-Trip Detection Logic and only applies to specific DTCs.
See also:
• WIRING DIAGRAMS
• THEORY & OPERATION