OBD II System

OBD-II SYSTEM
The OBD-II (On Board Diagnostic - Generation II) system affects all 1995 and later model year vehicles using a new or redesigned control processor. It is the result of regulations passed by the California Air Resources Board (CARB), and adopted by the US Federal Government (in lieu of developing it's own regulations) through at least the 1998 model year.

The OBD-II system is a high-tech system designed to combine the functions of both the conventional ECM (Engine Control Module) and TCM (Transaxle Control Module). The system monitors or controls the real-time operating conditions of the engine (from the intake air temperature to the exhaust composition) and transmission (from range selection setting to gear ratio). The OBD-II system utilizes the same sensors and control devices as the conventional ECM/TCM systems, as well as some new ones to monitor systems that were not monitored before. All the inputs are combined into the PCM (Powertrain Control Module). In some cases, the actual signal is replaced periodically by a test signal to verify that the sensor is functioning properly.

Should a powertrain problem occur, or one of the operating parameters drift out of tolerance, the OBD-II system will record an appropriate diagnostic trouble code for the malfunction. This diagnostic trouble code, upon retrieval, will help identify the cause of the trouble with a minimum of time/effort expended, thus lowering the cost and time of repairing the vehicle, and ensuring that proper repairs are made.

The significant parameters and requirements imposed with the OBD-II system are as follows:

^ The MIL (Malfunction Indicator Lamp) may only illuminate when a problem that will increase emissions occurs, and may only flash when a misfire has occurred.

^ When a misfire is detected, the MIL must flash at a rate of one blink per second. Afterwards, the MIL must illuminate continuously.

^ The OBD-II system must be able to identify the exact cylinder that is misfiring, and relay this information to the technician.

^ RPM, closed or open loop, and load conditions occurring at the first misfire event are recorded and stored.

^ The system must be able to test for and detect fuel system evaporative system leaks of 0.040" or greater. When a leakage is detected, the information is stored and the MIL illuminates.

^ The entire fuel injection system must be continuously monitored and, if a fault is detected, a code must be stored and the MIL illuminated.

^ The catalyst system must be monitored once during each driving cycle, and then only when the engine enters closed loop operation, the vehicle speed is between 20 and 50 MPH, and the engine is under medium load conditions. To pass, the exhaust must have at least 50% less HC concentration at the output of the converter compared to the input.

^ The catalyst monitoring system must be able to determine if a diagnostic trouble code has been manually cleared, but the converter hasn't been replaced.

^ The OBD-II system requires general monitoring of the fuel delivery system, and specific monitoring of the front Heated Oxygen Sensor (for current flow and voltage drop) and the Rear Heated Oxygen Sensor (catalyst monitoring - for voltage and response rates).

^ An EGR (Exhaust Gas Recirculation) system fault (both low and high flow rates) must cause a code to be stored, and the MIL illuminated.

^ All input devices must be monitored to ensure all circuits are intact and delivering signals that are reasonable when compared to other input device signals being received at that time.

^ All output devices must be monitored to ensure that they respond appropriately to controller commands.

^ The Engine Coolant Temperature (ECT) Sensor signal must be monitored to verify that closed loop operation occurs within an acceptable time frame.