On-Board Diagnostics II Overview

ON BOARD DIAGNOSTICS II
OVERVIEW:
On Board Diagnostic (OBD) is a diagnosis system integrated within the Engine-Management ECM, where it continually monitors emissions-relevant components. Any malfunction that occurs is recognized, recorded and displayed to the driver via a Malfunction Indicator Lamp (MIL).

OBD II represents the second generation of diagnosis-compatible engine management systems. Compared to periodic inspections conducted at fixed intervals, OBD II furnishes the following advantages:
- Continuous monitoring of exhaust emissions.
- Prompt early warning of malfunctions.
- Advanced diagnostic options designed to facilitate fault diagnosis and rectification during vehicle servicing.

Long-term objectives include incorporation of a feature to allow malfunction detection in the emissions-control system and the resulting deterioration in exhaust emissions in the course of roadside checks using a simple scan tool.




Component malfunctions within the engine-management system can lead to substantial increases in a vehicle's pollutant emissions. Because it would entail using complex technical procedures, the system does not directly measure concentrations of:
CO - carbon monoxide
HC - hydrocarbons
NOx - nitrous oxides
Instead, the data are derived from examination of the emissions-relevant components in the engine-management system. Another advantage of this procedure is that it makes it possible to use the scan tool to detect malfunctions.

REQUIREMENTS:
- Standardized diagnosis socket located in the vicinity of the driver.
- Standardized Diagnostic Trouble Codes (DTC) for all manufacturers.
- DTC display with a commercially-available diagnosis tool.
- Display of operating conditions under which a malfunction occurs.
- Definitions of when and how emissions-relevant errors must be displayed.
- Standardized designations and acronyms used to identify components and systems.




OBJECTIVES:
- Operational monitoring of all components exercising an effect on exhaust emissions.
- Protection of the catalytic converter.
- Visible display (malfunction indicator lamp) signals malfunctions in emissions-relevant components.
- DTC memory to store Diagnostic Trouble Codes.
- Diagnostic feature.

To achieve these objectives, the engine's electronic control unit monitors the following components and systems:
- The catalytic converter
- The oxygen sensors
- Misfire recognition
- Secondary air injection
- Exhaust-gas recirculation, and
- Evaporative emissions with leak check
- Fuel supply and distribution system
- Automatic transmission
- All sensors and DTM components connected to the ECM.

NOTE: Because the operating response of the transmission also affects the quality of the exhaust emissions, its ECM must be included in the diagnosis readout.

OBD II represents an advanced version of OBD-I
OBD-I Monitors:
- The operational status of all input and output components based on measurements of voltage drop at the components themselves.

OBD II Monitors:
- All of the input and output functions monitored by OBD I, such as:
- Short to positive
- Short to ground
- Open circuits
- Plausibility of signals and component status for emissions-relevant functions (e.g., catalytic converter, oxygen sensor).
- System operations (e.g., secondary air injection).
- The entire drivetrain (e.g., automatic transmission backup function).

TERMINOLOGY:
- CARB (California Air Resources Board): Agency responsible for maintaining air quality in California.
- SAE (Society of Automotive Engineers): This professional organization formulates recommendations and guidelines for implementation of legal requirements (e.g., standards).
- NLEV (Non-Low Emission Vehicles): Homologation classification for vehicles conforming to currently applicable regulations (0.25 g/mi HC).
- TLEV (Transient Low Emission Vehicles): Homologation classification for vehicles with low emissions (0.125 g/mi HC).
- LEV (Low Emission Vehicles) Homologation classification for vehicles that comply with the new and more stringent requirements (0.075 g/mi HC).
- ULEV (Ultra Low Emission Vehicles): Homologation ratings for vehicles characterized by supplementary reductions in emissions (0.04 g/mi HC).
- ZEV (Zero Emission Vehicles): Homologation classification for vehicles generating no pollutant emissions.
- Generic Scan Tool (GST): Universal tester employed to read error codes from the fault memory.
- ISO 9141-CARB: Standard regulating data transfer to the readout device.
- Comprehensive-Components-Diagnose: Diagnosis system designed to check operation of all electrical components and output circuitry by monitoring voltage drop at the respective components.
- Driving Cycle: Test cycle in which the engine is started and a particular diagnosis function processed, cycle terminates when the engine is subsequently switched off.
- Trip: Driving cycle following a period with the engine at rest; all components and operations are tested in this process.
- FTP72 (Federal Test Procedure): A test cycle defined for the USA, lasting 1372 seconds and covering a distance of 7.5 miles. The maximum vehicle speed is 91.2 km/h.
- NOx (oxides of nitrogen): Compounds of nitrogen and oxygen. The NOx component in motor-vehicle exhaust gases originates from atmospheric nitrogen when fuel is burned during high-temperature, high-pressure combustion within the engine.
- CO (carbon monoxide): This gas is generated when hydrocarbons combust in an oxygen-deficient environment.
- HC (hydrocarbons): When used in discussions of exhaust systems this term refers to the residual unburned hydrocarbons in the exhaust gases.
- Stoichiometric: In automotive applications, the term stoichiometric describes the ideal mass ratio of intake air to fuel in the air/fuel mixture. This is the mixture at which fuel would be completely combusted without generation of unburned residual components and by-products (such as carbon monoxide).
- Readiness Code: 8-digit binary code indicating whether the engine-management system has completed all emissions-relevant diagnosis operations.
The readiness code is generated:
- Run through all diagnostics one time free of malfunctions (MIL does not light up).
- All diagnostics have been run through at least twice and recognized DTC's stored in DTC memory and indicated with MIL light lit up.
Readiness code is erased only when:
- DTC memory is erased
- disconnecting the battery or the engine control module.
- MIL (Malfunction Indicator Lamp): This warning lamp lights up to indicate that the engine-management system has detected a malfunction in an emissions- relevant component. Depending upon the type of malfunction and its specific individual display protocol, the control module may respond to the malfunction by initiating a continuous or flashing display:
- Immediately,
- After 2 trips, or
- Upon completion of 2 driving cycles.
There is also a category for malfunctions that are stored in the DTC memory without triggering an MIL display.