Automotive OBD for Continuous Vehicle Monitoring

Date: June 29, 2020

Author: Akhila Nagaruru

OBD stands for On-board diagnostics. OBD system refers to the in-vehicle system, which monitors and reports the status of the vehicle. With the help of OBD port and OBD Connector provides access to the various vehicle parameters like vehicle speed, emission data, engine data in the form of Diagnostic Trouble codes are possible. Diagnosis solves almost every issue with in the vehicle. A standardized system that allows external electronic devices to interface with the car’s computer system. The OBD system is designed to reduce emissions by monitoring the performance of Engine components.

OBD system consists of ECU, Sensors, Actuators. ECU uses input from the various sensors to control the actuators to get the desired performance.MIL stands for Malfunction Indicator light, also known as Check Engine light, represents the early warning of malfunction to the vehicle driver. A modern vehicle supports hundreds of parameters that can be accessed through the Diagnostic Link Connector (DLC) using a Scan tool.

What are OBD-I and OBD-II?

There are two kinds of OBD systems:

OBD-I: OBD systems developed throughout the 1980s are referred to as OBD-I.OBD-I systems are not standardized. It is difficult for a mechanic who wants to access diagnostic information has to buy a different tool for different vehicles.

OBD-2: In the early 1990s, SAE (Society of Automotive Engineers) and ISO issued standards for the interchange of digital information between ECUs and diagnostic scan tools. All OBD-II based vehicles are required to use standard diagnostic connectors and communicate via one of the standard OBD-II protocols. OBDII cars have a port under the dashboard on the driver’s side where the OBD connector is plugged in and, the owner gets the information regarding repairs of the vehicle subsystem. OBD provides access to status information for power Engine, Emission Control Systems, Vehicle identification number, Calibration Identification number, Ignition counter, Emissions Control system counters.

Types of Vehicle communications

Vehicle communication classified into two types

  1. In-vehicle networking

Exchange of data between the various Electronic control units within the vehicle network.
Protocols like CAN, LIN, Flexray are used for in-vehicle communication.

  1. OBD II protocol and Vehicle Diagnostic Communication

The California Air Resources Board (CARB) mandated all cars needed OBD capability and should follow the CARB initiated protocol mandates. CARB guidelines to be followed for providing data storage and data accessibility to the external scan tools.

Vehicle diagnostics communication is used for the diagnosis of vehicle subsystems.

OBD system internally monitors various subsystems and reports if any malfunction is detected through vehicle diagnostics communication. It is a query-response communication. The external scan tool is connected to the internal vehicle system through the OBD port and OBD connector. The External tool requests for the information, and ECU responds with the answer. CAN, LIN, Flexray, MOST are used for diagnostic communication. Additionally, there are K-Line, J1850, KWP200, UDS, and more that work on the serial protocol is also used for this communication.

When sensors detect any malfunction, they trigger a message in the form of trouble code. With the help of Scan, tool users are allowed to check these trouble codes and understand the exact problem inside the vehicle, and the triggered messages are deleted from the computer’s memory once the problem is resolved. The problem is resolved by reconfiguring and reflashing the ECU. Software plays an important role in fixing problems and to enhance the existing features.

What are the Diagnostic Trouble Codes?

Information sent to the Scan tool is in the form of Diagnostic Trouble codes. Trouble codes start with a letter and include four or five numbers that represent a particular vehicle subsystem and the problem experienced by it. OBD Scan tools are preloaded with definitions for these codes. Apart from these generic codes, Individual manufacturers have their specific codes, and these codes are not released to the public directly.

Fault codes:

  • PXXXX: Indicates problem in the powertrain (Engine and transmission ECU)
  • UXXXX: Indicates problem in the network
  • BXXXX: Indicates problem in the body
  • CXXXX: Indicates problem in the chassis

Why is OBD needed?

  • Help Technicians properly diagnose and repair complex problems.
  • A robust Emission control system
  • Effective and inexpensive emission inspections.
  • Keep emissions low by detecting any problem in the emission control system in prior.
  • Supports for the good lifetime period of vehicle and driver safety
  • It eliminates the replacement of any vehicle subsystem by early detection of problems
  • Prevent secondary Malfunctions
  • Eliminates unnecessary repairs since the scan tool gives correct information where the problem exactly occurred.

OBD Diagnostic Procedure

  • The system waits for the monitoring conditions.
  • Observes signals coming directly from the component or related to the performance of the component
  • Verifies functionality, i.e., checks whether it satisfies the malfunction criteria
  • Notifies owner of the vehicle about the fault by MIL illumination, storing fault code

Why was the Diagnostic system developed initially?

After Electronic fuel injection came into existence, the concept of a connected car started. OBD was initially developed to regulate emissions by controlling the engine and mass adoption of electronic fuel injectors.

Limitations of the OBD system

  • Data collected through the OBD connector is related to the powertrain and Emission ECU. It does not provide any access to the body control data seat belt status, airbag status.
  • Supports only query-response communication
  • OBD provides only read access to the emission-related data. No write access for reprogramming, and hence it is difficult to rectify the malfunction observed.

OBD Software Stack:

The International Standards Organization introduced ISO 15031 standard for Communication between vehicle OBD port/connector and external scan tool for emissions-related diagnostics. OBD II Stack is a set of software services for the implementation of diagnosis based on ISO 15031 standard. Stack suits for light commercial vehicles and passenger cars. OBD II stack is implemented over five signalling protocols like CAN, J1850 VPW, J1850 PWM, ISO 9141, KWP 2000.

OBD Testing

Pre- and post-production testing should be done by the vehicle manufacturers. Both manufacturer self-testing and agency testing should be done.

Manufacturer Self-Testing

  • Testing used to detect any malfunctions before emissions exceed threshold value is known as Demonstration Testing.
  • Ensuring vehicles properly communicate required information according to the standards
  • Manufactures have to implant faults and verify whether the system detects the fault.
  • Tracks how frequently the system monitors really while the car is running on the road.

Agency Testing:

  • Confirmatory testing to verify whether malfunctions are detected before emissions exceed the threshold value
  • Actual in-use vehicle testing with implanted faults, data logging, on-road driving
  • Testing the right data is stored after detecting a malfunction.
  • Testing whether monitors run when they are supposed to

Difference between On-board and off-board diagnostics

On-board diagnostics

On-board diagnostics completely dedicated to emission-related diagnostics.MIL is associated with OBD to indicate any urgency emission-related issues and stores the fault code. Parameter IDs (PIDs) are sent to ECU through the tester tool to request the data from the particular vehicle ECU.PIDs have been specified by SAE J1979 and few by automotive OEMs. Services provided by OBD-II are categorized based on nodes, and these services refer to the functions of the OBD2 software stack. Hence every node has some PIDs.

Functions of each OBD-II node:

  • Mode $01: Request live data from the powertrain
  • Mode $02: Request Freeze frames (when the issue occurred)
  • Mode $03: Request Stored DTC, displays 4-digit code to identify the error
  • Mode $04: clear stored emission-related data
  • Mode $05: Requests oxygen sensors test results
  • Mode $06: Requests test results from exhaust gas sensors, catalyst monitor, fuel system monitor, and accessed by only professional tester tool
  • Mode $07: Request pending Trouble codes
  • Mode $08: Request control of On-board systems by an off-board diagnostic system
  • Mode $09: Request vehicle information like vehicle identification number etc
  • Mode $0A: Request Permanent trouble codes stored in non-volatile memory

Off-board diagnostics

Off-board diagnostics provide diagnostic services of all other ECUs other than emission. There are several protocols defined for off-board diagnostics. But UDS (Unified Diagnostic Service) protocol is the most popular diagnostic protocol. Diagnostics manager of UDS protocol is not limited to store fault codes but also capable of reprogramming, writing data on the ECU. Off-board diagnostics work when the vehicle is stopped as well as moving. Contrast with off-board diagnostics lies in the reporting part, an instant fault is communicated through the MIL system in On-board diagnostics, no such instant communication service is available in off-board diagnostics. It stores the issue in the EEPROM part of ECU, and information can be retrieved in the garage.

When the UDS stack is integrated to the vehicle ECU, UDS services are configured to it. UDS requests, i.e., service ID can be sent to the controllers to get the response, which can be either positive or negative. This helps to get the operating conditions of the ECU and be able to diagnose a fault.

Some of the UDS service:

  • 0x10: Diagnostic session control - UDS uses different operating sessions, which can be changed using Diagnostic session control. Depending on the active session, different services are available. The control unit is in the default session in the starting.
    1. Programming session used to upload software.
    2. An extended diagnostic session is used to unlock additional diagnostic services.
    3. A safety system diagnostic session is used to test all safety-related diagnostic functions.
  • 0x11: ECU reset - This service is used to restart the control unit. A hard reset, key off on Reset, Soft reset are the different services provided.
  • 0x27: Security access - Used to enable security-critical services.

OBD Dongle

OBD dongle is an adapter used to connect to the OBD port and provides diagnostic data to the Bluetooth connected computing device, i.e., smartphone, telematics control unit. It acts as an interface between the computer and ECU and translates messages from one protocol to another.

OBD2 Data Logger

Unlike OBD2 dongles and OBD2 Scanners, after requesting for the information by sending PIDs, OBD2 data Loggers log data into the SD card. Data may be used to optimize driving by noticing vehicle speed, RPM, throttle position, etc data can be logged. But data is in raw format and can be converted to a human-readable format by OEM conversion rules.

Illustration of how OBD Functions in the Catalyst Monitoring system

The exhaust and catalyst converter is used to safely move the exhaust gases away from the engine. OBD II equipped vehicles use upstream and downstream oxygen sensors, which are used to measure the oxygen content in the exhaust gases. These sensors are engine management feedback sensors that measure how efficiently fuel and air were burned in the combustion chamber. Exhaust gases coming out from the combustion chamber are sent to the catalyst combustion chamber, where these harmful gases are converted into harmless by absorbing nitrogen content and releases free oxygen. Downstream Oxygen sensors located behind the catalyst converter monitor converter efficiency. If the efficiency is less, the converter is not cleaning up the pollutants in the exhaust. The final emission of these exhaust gases through tailpipe to the atmosphere.

Oxygen sensor data is used to evaluate catalyst conversion performance. Based on these oxygen sensor data, one can correlate the exhaust tailpipe emissions with the catalyst system performance. When emissions exceed malfunction threshold, the OBD system turns on MIL and store fault information of the catalyst.

Further reading

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