OBD II System

The goal of the On Board Diagnostics-II (OBD-II) regulation, which was mandated by the Environmental Protection Agency (EPA), is to ensure proper emission system operation for the life of the vehicle by providing the vehicle with an "on-board" diagnostic system which is capable of continuously monitoring the efficiency of the emission control system. To achieve this goal, OBD-II diagnostics monitors emissions-related components and systems for deterioration and identifies an emissions-related malfunction before the driver notices a problem with the vehicle. The Engine Control Module (ECM) monitors many different circuits in the fuel injection, ignition, emission and engine systems. If the ECM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code in the ECM memory. If the code applies to a non-emissions related component or system and the problem is repaired or ceases to the exist, the ECM cancels the code after a predetermined number of drive cycles. Certain criteria must be met before the ECM stores a DTC in memory. The criteria may be a specific range of engine RPM, engine temperature, and/or input voltage to the ECM. keep in mind though that the ECM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the DTC criteria for the circuit has not been met. For example, lets SAY THE DTC criteria requires the ECM to monitor the circuit only when the engine operates between 750 and 2000 RPM.

Suppose the sensors output circuit shorts to ground as a result of engine torque when the engine operates above 2400 RPM (resulting in 0 volt input to the ECM). Since the condition happens at the engine speed above the maximum threshold, in this case 2000 RPM, the ECM will not store a DTC.

As a functional test, the MIL (check engine) illuminates at key ON before engine cranking. Whenever the ECM sets a DTC that affects vehicle emissions, it sends a signal to illuminate the MIL. There are some monitors that may take consecutive trips, with a detected fault, before the MIL is illuminated. This is also known as "PENDING" codes. Also, m the MIL flashes when the ECM detects an active engine misfire, then shuts off fuel to the affected cylinder. OBD-II diagnostics supports not only the standard test mode of OBD-II (Mode 01 - Mode 07) but also an advanced test Mode (Mode 22: Supported PID-analog inputs and outputs, digital inputs and outputs). All of these diagnostic test modes are supported using Hi-Scan Pro through the DLC.

The ECM executes the following key monitoring for OBD-II:
- Primary emission control system/components
- Heated oxygen sensors (includes heater)
- Catalyst
- Fuel system
- Evaporative emission system
- Misfire
- Comprehensive components (input and output)