All modern vehicles, equipped with internal combustion engines, contain several systems/components which have the specific purpose of reducing toxic exhaust gas emissions. This is mandatory because every new vehicle sold needs to be compliant with the current emission limit legislation (e.g. SULEV, Euro 6, etc.)
The engine systems and components, which have impact on exhaust gas emissions, must be monitored continuously to make sure that the toxic emission limits required by law/legislation are achieved on a daily basis.
OBD (On-board Diagnostics) is a set of rules, software and hardware, with the specific purpose of monitoring the powertrain components/systems, whose functionality has an impact on exhaust gas toxic emissions levels. With other words, if a specific powertrain component, in case of a defect/failure, leads to increased toxic exhaust gas emission levels, it must be OBD monitored.
Image: Malfunction Indicator Lamp (MIL)
The OBD standard specifies that the driver must be warned if there is a failure with any system/component which has impact on the level of toxic exhaust gas emissions. If such a failure occurs, the driver is informed through the MIL lamp on the dashbord.
There are two types of MIL lamps: an icon with the shape of an engine, and an icon with the message “SERVICE ENGINE SOON”. Both have the same meaning, they inform the driver about an OBD related system/component failure.
History of On-Board Vehicle Diagnostic
Starting with 1980, many new vehicles were equipped with Electronic Control Units (ECU), mainly for engine fuel control. As the complexity of the electronic control systems increased, vehicle manufacturers started to add on-board diagnostic functions into the ECU software.
Early diagnostics were mainly for the electrical circuits (open circuit, shortcut to ground, etc.) of the sensors and actuators. The detected failures were reported as fault codes using a series of voltage pulses. Each fault code was referenced to a particular sensor, actuator or circuit.
These diagnostic procedures were initially put in place to help the service technicians identify the failed components. The drawback was that they were not standardized, each manufacturers had their own fault codes and test procedures.
In April 1985, California Air Resource Board (CARB) adopted the first OBD requirements. These are know as the first generation of on-board diagnostic procedures, OBD-1. Vehicles needed to be compliant with OBD-1 requirements starting with 1988 model year.
The purpose of OBD-1 was to define what on-board diagnostic tests needs to be performed in an ECU, in order to identify failed components, which have impact on exhaust gas emission levels. The OBD-1 standard also stated that the driver must be aware of failed components through the activation of the MIL lamp in the dashboard. The OBD-1 standard imposed also the storage of the fault codes in the ECU memory for a later reading.
Compared to current, modern diagnostic requirements, OBD-1 standard was fairly simple. It requested the monitoring of the emission-related electric components for open circuit and shortcut to ground/battery. It also contained performance monitoring for the following components:
- ECU
- Fuel injection system
- Ignition system
- Exhaust Gas Recirculation (EGR) system (if equipped)
In 1992, after close collaboration with vehicle manufacturers, Environmental Protection Agency (EPA) and Society of Automotive Engineers (SAE), CARB adopted the revised version of the OBD regulations. This new version, known as OBD-2, applied to all newly sold passenger vehicles (gasoline, diesel and alternative fuel) starting with 1996.
The European version of OBD-2 is named EOBD and basically contains the same rules and regulations as OBD-2. EOBD became mandatory for vehicle homologation starting with 2001 for gasoline powered vehicles, and with 2004 for diesel engine vehicles.
For simplification, since the latest applicable standards are OBD-2 for US and EOBD for EU, we are going to use the general abbreviation OBD for both of them.
Which are the OBD monitoring functions ?
OBD regulation states that all systems and components which are related to exhaust gas emission levels, must be monitored if, in case of a malfunction, failure or defect, they can cause an increase in toxic exhaust gas emissions.
The component monitoring is going to be performed in the appropriate ECU, by means of software functions. The result of the monitoring function must be reported to an external diagnostic device (scantool) in the format specified by the standard.
Depending on the type of engine, the OBD regulations demands the following components to be monitored (inside the engine control software):
| Gasoline Engine | Diesel Engine |
| Catalyst Monitoring | Non-Methane Hydrocarbon (NMHC) Converting Catalyst Monitoring |
| Heated Catalyst Monitoring | Oxides of Nitrogen (NOx) Converting Catalyst Monitoring |
| Misfire Monitoring | |
| Evaporative System Monitoring | Crankcase Ventilation (CV) System Monitoring |
| Secondary Air System Monitoring | NOx Adsorber Monitoring |
| Fuel System Monitoring | |
| Exhaust Gas Sensor Monitoring | |
| Exhaust Gas Recirculation (EGR) System Monitoring | |
| Positive Crankcase Ventilation (PCV) System Monitoring | Boost Pressure Control System Monitoring |
| Engine Cooling System Monitoring | |
| Cold Start Emission Reduction Strategy Monitoring | |
| Air Conditioning (A/C) System Component Monitoring | |
| Variable Valve Timing and/or Control (VVT) System Monitoring | |
| Direct Ozone Reduction (DOR) System Monitoring | Particulate Matter (PM) Filter Monitoring |
| Comprehensive Component Monitoring | |
| Other Emission Control or Source System Monitoring | |
Detailed information about each monitoring function can be found in the standards:
- CARB: “Malfunction and Diagnostic System Requirements–2004 and Subsequent Model-Year Passenger Cars, Light-Duty Trucks, and Medium-Duty Vehicles and Engines”
- EC (European Commission): “DIRECTIVE 98/69/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 13 October 1998 relating to measures to be taken against air pollution by emissions from motor vehicles and amending Council Directive 70/220/EEC”
OBD communication protocols
OBD regulations are defined by a series of SAE/ISO standards, that describe in detail how certain OBD functions need to be implemented on the ECU side, and how the communication between the ECU and the diagnostic tool is going to be performed.
OBD-2 (US) regulations ar described in SAE standards, EOBD (EU) regulations are described in ISO standards.
The OBD communication protocol can be regarded as an OSI (Open Systems Interconnection) model, with several layers, each defined in a standard:
| Applicability | OSI (7 layers) | Emission-related Diagnostic | |||
| Communication protocol | K – line | KWP | PWM / VPW | CAN | |
| Seven Layers according to ISO/IEC 7498 and ISO/IEC 10731 | Physical (Layer 1) | ISO 9141-2 | ISO 14230-1 | SAE J1850 | ISO 11898 ISO 15765-4 |
| Data Link (Layer 2) | ISO 9141-2 | ISO 14230-2 | SAE J1850 | ISO 11898 ISO 15765-4 | |
| Network (Layer 3) | – | – | – | ISO 15765-2 ISO 15765-4 | |
| Transport (Layer 4) | – | – | – | – | |
| Session (Layer 5) | – | – | – | ISO 15765-4 | |
| Presentation (Layer 6) | – | – | – | – | |
| Application (Layer 7) | SAE J1979 ISO 15031-5 | SAE J1979 ISO 15031-5 | SAE J1979 ISO 15031-5 | SAE J1979 ISO 15031-5 | |
From the powertrain egineering point of view, the Physical layer (1) and the Application layer (7) are most important, because they define the medium for communication (KWP, CAN, etc.) and the content of the monitoring functions.
OBD means also data exchange between vehicle and external equipment (scantool). This can be done using on of the following protocols:
| Protocol | Description |
| SAE J1850 PWM |
|
| SAE J1850 VPW |
|
| ISO 9141-2 |
|
| ISO 14230 (KWP2000) |
|
| ISO 15765 (CAN) |
|
The implementation of one protocol or another, for OBD communication, is somehow linked to the emission regulations. More stringent emission regulations (e.g. Euro 6) have increased the number of components and systems to be monitored.
Due to the increase of data to be monitored and exchanged with the diagnostic tool, the usage of a faster communication protocol becomes mandatory. In the table below you can see some examples of vehicles with the associated emission type approval and communication protocol for OBD:
| Vehicle | Engine | Emission level | Communication Protocol |
| Renault Clio | 1.5 dCi 65 CP | Euro 3 | ISO 14230-4 (KWP FAST) |
| Renault Clio | 1.6 MPI 90 CP | Euro 4 | ISO 14230-4 (KWP 5BAUD) |
| Renault Megane 2 | 1.5 dCi 85 CP | Euro 4 | ISO 15765-4 (CAN 11/500) |
| Mini Cooper | 1.6 122 CP | Euro 5 | ISO 15765-4 (CAN 11/500) |
| Opel Zafira EcoFlex | 1.7 CDTI 125 CP | Euro 5 | ISO 15765-4 (CAN 11/500) |
| Skoda Fabia | 1.6 TDI 90 CP | Euro 5 | ISO 15765-4 (CAN 11/500) |
The Application layer standards define the communication between vehicle and external equipment for emissions-related diagnostics, as well as the specification for the diagnostic connector and related electrical circuits.
For OBD-2 (US) vehicle the Application layer is defined in the SAE standards, for EOBD (EU) vehicles, in the ISO standards.
In the table below is a detailed structure of each standard and their equivalent ISO/SAE:
| ISO | SAE | ||
| Standard | Description | Standard | Description |
| ISO 15031-1 | General information | none | none |
| ISO 15031-2 | Terms, definitions, abbreviations, and acronyms | SAE J1930 | Electrical/Electronic Systems Diagnostic Terms, Definitions, Abbreviations and Acronyms |
| ISO 15031-3 | Diagnostic connector and related electrical circuits, specification and use | SAE J1962 | Diagnostic Connector |
| ISO 15031-4 | External test equipment | SAE J1978 | OBD-2 Scan Tool |
| ISO 15031-5 | Emissions-related diagnostic services | SAE J1979 | E/E Diagnostic Test Modes |
| ISO 15031-6 | Diagnostic trouble code definitions | SAE J2012 | Recommended Practice for Diagnostic Trouble Code Definitions |
| ISO 15031-7 | Data link security | SAE J2186 | E/E Data Link Security |
Which vehicle electronic control modules have to contain OBD monitoring functions ?
Most of the OBD monitoring strategies are embedded into the Engine Control Module (ECM). They can also be embedded in other control modules if they control systems that might have an impact on exhaust gas toxic emission levels.
For example, the Transmission Control Unit (TCU) could have OBD monitoring functions since a defect on a gear actuator can lead to increased emissions. This is valid because on homologation cycle, if the vehicle can not engage for example the 3rd gear, the engine will run at a less efficient operating point (too high or to low torque/speed). Thus, the gear actuator failure can cause increased exhaust gas toxic emission, so that is why it should be OBD monitored.
What is the difference between exhaust gas emission type approval limits and OBD limits ?
When a vehicle is homologated for a particular type approval emission limit (e.g. Euro 6) it means that during the homologation cycle (e.g. NEDC, WLTC, etc.), the level of toxic emission from the exhaust gas should not exceed the limits set by the legislation (see table below).
Gasoline (SI – Spark Ignition) engines
| Unit | Emission limits (type approval) | |||||||
| Emissions | mg/km | Euro 1 | Euro 2 | Euro 3 | Euro 4 | Euro 5a | Euro 5b/b+ | Euro 6 |
| THC | – | – | 200 | 100 | 100 | 100 | 100 | |
| NMHC | – | – | – | – | 68 | 68 | 68 | |
| NOx | – | – | 150 | 80 | 60 | 60 | 60 | |
| CO | 2720 | 2200 | 2300 | 1000 | 1000 | 1000 | 1000 | |
| PM | – | – | – | – | 5 | 4.5 | 4.5 | |
| PN | Nb/km | – | – | – | – | – | – | 6.0 · 1011 |
Diesel (CI – Compression Ignition) engines
| Unit | Emission limits (type approval) | |||||||
| Emissions | mg/km | Euro 1 | Euro 2 | Euro 3 | Euro 4 | Euro 5a | Euro 5b/b+ | Euro 6 |
| HC+NOx | 970 | 700 | 560 | 300 | 230 | 230 | 170 | |
| NOx | – | – | 500 | 250 | 180 | 180 | 80 | |
| CO | 2720 | 1000 | 640 | 500 | 500 | 500 | 500 | |
| PM | 140 | 80 | 50 | 25 | 5 | 4.5 | 4.5 | |
| PN | Nb/km | – | – | – | – | – | 6.0 · 1011 | 6.0 · 1011 |
Legend:
THC – Total HydroCarbons
NMHC – Non-Methane HydroCarbons
HC – HydroCarbons
NOx – Nitrogen Oxides
CO – Carbon Oxides
PM – Particulate Mass
PN – Particulate Number
OBD emission levels are higher the type approval limits (see table below). These limits are used to decide if a particular component/system failure should lead to the activation of the MIL.
| Unit | OBD limits (MIL ON if exceeded) | ||||
| Emissions | mg/km | Euro 5 | Euro 6 | ||
| Engine | Gasoline | Diesel | Gasoline | Diesel | |
| NMHC | 250 | 320 | – | 320 | |
| NOx | 300 | 540 | – | 240 | |
| CO | 1900 | 1900 | – | 1900 | |
| PM | 50 | 50 | – | 50 | |
MIL activation criteria
The OBD limits are used to decide if the MIL should be ON or OFF in case of a component failure. The rules are as following: if a failure occurs and the emission level of the vehicle are above the type approval limits, a diagnostic trouble code (DTC) must be stored in the ECU memory; if the emission levels, for the same failure, exceed also the OBD limits, together with the DTC storage, the MIL should be ON.
Image: Malfunction Indicator Lamp (MIL) activation criteria
The vehicle manufacturers must perform several tests to measure the level of additional exhaust emissions for each failure type. Based on this analysis the engine control module will be programmed to activate the MIL depending on the severity (emission level) of the failure.
If the failure disappears (e.g. loose electrical contact) for whatever reason, the diagnostic trouble code must be kept in the ECU memory for 40 engine cycles (the definition of the engine cycle is in the OBD standard). If the MIL was ON, for the same failure, it will be set OFF after 3 fault-free engine cycles.
Detailed analysis of the OBD monitoring functions and diagnostic procedures are subject for future articles.
For any questions or observations regarding this tutorial please use the comment form below.
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Amar D
Its a very good knoledgeble article which you explained in very simple wording which is imp.
Thank you for article.