Introduction to On-Board Diagnostics (OBD)

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.

Malfunction Indicator Lamp (MIL)

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 EngineDiesel Engine
Catalyst MonitoringNon-Methane Hydrocarbon (NMHC) Converting Catalyst Monitoring
Heated Catalyst MonitoringOxides of Nitrogen (NOx) Converting Catalyst Monitoring
Misfire Monitoring
Evaporative System MonitoringCrankcase Ventilation (CV) System Monitoring
Secondary Air System MonitoringNOx Adsorber Monitoring
Fuel System Monitoring
Exhaust Gas Sensor Monitoring
Exhaust Gas Recirculation (EGR) System Monitoring
Positive Crankcase Ventilation (PCV) System MonitoringBoost 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 MonitoringParticulate 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:

ApplicabilityOSI (7 layers)Emission-related Diagnostic
Communication protocolK – lineKWPPWM / VPWCAN
Seven Layers
according to
ISO/IEC 7498
ISO/IEC 10731
Physical (Layer 1)ISO 9141-2ISO 14230-1SAE J1850ISO 11898
ISO 15765-4
Data Link (Layer 2)ISO 9141-2ISO 14230-2SAE J1850ISO 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:

  • Pulse Width Modulation
  • used mainly by Ford Motor Company
  • data transfer speed 41.6 kBaud/s
  • uses two wires for data transmission
  • Variable Pulse Width
  • used mainly by General Motors
  • data transfer speed between 10.4 and 41.6 kBaud/s
  • uses two wires for data transmission
ISO 9141-2
  • used mainly by Chrysler, European and Asian vehicle manufacturers
  • data transfer speed 10.4 kBaud/s.
  • uses only one wire for data transmission (K-line), the second line (L-line) is optional
ISO 14230 (KWP2000)
  • KeyWord Protocol
  • similar with ISO 9141-2
  • data transfer speed between 1.2 and 10.4 kBaud/s.
  • uses only one wire for data transmission (K-line), the second line (L-line) is optional
ISO 15765
  • Controller Area Network
  • starting with 2008 all new vehicles sold in USA must use CAN as OBD communication protocol
  • starting with Euro 4 most of the vehicles sold in EU use CAN as OBD communication protocol
  • data transfer speed between 250 and 500 kBaud/s
  • uses two wires for data transmission (CAN-high and CAN-low)

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:

VehicleEngineEmission levelCommunication Protocol
Renault Clio1.5 dCi 65 CPEuro 3ISO 14230-4 (KWP FAST)
Renault Clio1.6 MPI 90 CPEuro 4ISO 14230-4 (KWP 5BAUD)
Renault Megane 21.5 dCi 85 CPEuro 4ISO 15765-4 (CAN 11/500)
Mini Cooper1.6 122 CPEuro 5ISO 15765-4 (CAN 11/500)
Opel Zafira EcoFlex1.7 CDTI 125 CPEuro 5ISO 15765-4 (CAN 11/500)
Skoda Fabia1.6 TDI 90 CPEuro 5ISO 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 15031-1General information nonenone
ISO 15031-2Terms, definitions, abbreviations, and acronymsSAE J1930Electrical/Electronic Systems Diagnostic Terms, Definitions, Abbreviations and Acronyms
ISO 15031-3Diagnostic connector and related electrical circuits, specification and useSAE J1962Diagnostic Connector
ISO 15031-4External test equipmentSAE J1978OBD-2 Scan Tool
ISO 15031-5Emissions-related diagnostic servicesSAE J1979E/E Diagnostic Test Modes
ISO 15031-6Diagnostic trouble code definitionsSAE J2012Recommended Practice for Diagnostic Trouble Code Definitions
ISO 15031-7Data link securitySAE J2186E/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

UnitEmission limits (type approval)
Emissionsmg/kmEuro 1Euro 2Euro 3Euro 4Euro 5aEuro 5b/b+Euro 6
PNNb/km6.0 · 1011

Diesel (CI – Compression Ignition) engines

UnitEmission limits (type approval)
Emissionsmg/kmEuro 1Euro 2Euro 3Euro 4Euro 5aEuro 5b/b+Euro 6
PNNb/km6.0 · 10116.0 · 1011

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.

UnitOBD limits (MIL ON if exceeded)
Emissionsmg/kmEuro 5Euro 6

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.

Malfunction Indicator Lamp (MIL) activation criteria

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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