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On-Board Diagnostic Systems

Basic Description:

On Board Diagnostic Scanner in useOn-board diagnostics in automobiles refers to a vehicle's built-in capability to monitor its own vital functions and record/report that information to the vehicle operator or repair technician. On-board diagnostic systems were first introduced in the 1980's in response to increasing government emissions regulations. Automotive manufacturers recognized the need to monitor certain vehicle systems so that the engine's operating parameters could be optimized for emissions control.

The first generation of on-board diagnostic systems is now known as OBD-I. OBD-I systems were relatively primitive and inconsistent between manufacturers. OBD-I regulations were aimed at getting manufacturers to produce better emission control systems. By denying vehicle registration for cars that were unable to meet the new emissions standards, the government hoped to entice consumers to buy cars that were known to have better emissions ratings. The OBD-I initiative never achieved the level of success that the government had intended because there were no standards imposed on manufacturers restricting the means by which the diagnostic information was transferred. This made obtaining reliable emissions data very difficult.

OBD-II was introduced in the mid 1990's. OBD-II systems are more sophisticated than their predecessors and are more robust in their engine monitoring and control capabilities. All vehicles manufactured after January 1, 1996 are required to have OBD-II systems; although some manufacturers implemented OBD-II as early as 1994. OBD-II standards were originally developed by SAE and later adopted by the EPA and CARB (California Air Resources Board).

OBD-II systems have greater functionality than OBD-I systems, but more importantly there is a much higher level of standardization between manufacturers. OBD-II systems have a standard diagnostic connector and they use standard electrical signal protocols and messaging formats. OBD-II makes it possible to obtain diagnostic information related to emissions controls from any vehicle with a single scanner.

There are essentially three different OBD-II protocols that are used by all manufacturers. Generally speaking, Chrysler along with all European and most Asian manufacturers use ISO 9141 circuitry. GM uses SAE VPM (Variable Pulse Width Modulation) communication patterns, and Ford uses SAE J1850 PMW patterns. Although all three communicate use the standard 16-pin, J1962 connector shown in the diagram, it is possible to differentiate between them by examining the pins in the connector socket. For example, systems that use the ISO 9141 protocol have a pin in the number 7 position and a pin in either the number 2 or number 10 position, while systems using the SAE protocol do not have connected pins in position number 7. 'OBD-II' in some cases has evolved to comply with OBD-II legislation and can also be referred to as 'EOBD' (European) or 'JOBD' (Japanese), which are effectively versions of OBD-II.

OBDII Connector Pin Layout

There are two basic ways that OBD-II systems can communicate diagnostic information to vehicle operators and technicians. The first way is via the "check engine light" or "MIL" (Malfunction Indicator Light) located on the dashboard display panel. The check engine light will either stay on until the detected problem is fixed or reset itself after a pre-determined number of successful engine starts with no error codes. The behavior of the light will depend on the severity of the problem. In any case, the information recorded by the vehicle's computer is stored until the code is cleared by a technician. The check engine light is very useful for spotting problems quickly, but it does not provide specific information about vehicle operation. More detailed diagnostics can be performed with off-board diagnostic scan tools. In response to either an illuminated check engine light or some detectable vehicle malfunction, an automotive technician can attach an OBD-II scan tool to the connector located under the dashboard on the driver side of the vehicle. The scan tool will decipher the error code stored in the vehicle's computer along with many other diagnostic signals to help the technician pinpoint the source of the malfunction. OBD-II scan tools communicate with the vehicle's ECM, which is essentially a computer that monitors the vehicle's operating conditions and adjusts the engine parameters accordingly. Depending on the type of scanner and the vehicle manufacturer, there can be as many as 300 diagnostic readings available.

The trouble codes that are required by law on the OBD-II are the same for all auto manufacturers. But each manufacturer has the freedom to include their own enhanced codes to provide information about additional electronic systems. These enhanced codes often cover areas that are not emissions related. For example, failures occurring in the ABS, HVAC, airbags and other body and electrical systems may be monitored and stored.

The second digit in the OBD-II trouble code is a zero if it is a generic (required) code or a 1 if it is an enhanced code. The third character in the code identifies the system where the fault occurred. The numbers 1 and 2 represent fuel or air metering problems, 3 indicates an ignition problem or engine misfire, 4 is for auxiliary emission controls, 5 indicates an idle speed control problems, 6 is for computer or output circuit faults, and 7 and 8 relate to transmission problems.

Time line



Oxygen Sensor
Engine Coolant Temp (ECT) Sensor
Intake Air Temperature (IAT) Sensor
Crankshaft Position (CKP) Sensor
Top Dead Center (TDC/CUP) Sensor
Map Sensor
Fuel Tank Pressure Sensor
Fuel Temperature Sensor
Oil Temperature Sensor
Wiper Washer Fluid Level Sensor
Vehicle Speed Sensor
Brake Fluid Level Sensor
Wheel Speed Sensor
Yaw Rate Sensor
Master Cylinder Pressure Sensor
Steering Angle Sensor
Throttle Position Sensor
Knock Sensor
Camshaft Position Sensor
Fuel Level Sensor


Malfunction Indication Lamp (MIL)

Data Communications
For More Information:
[1] OBD-II Systems, website.
[2] On-Board Diagnostics, Wikipedia.
[3] OBD-II PIDs, Wikipedia.
[4] On-Board Diagnostics(OBD), U.S. Environmental Protection Agency.
[5] California On-Board Diagnostics Program, California Air Resources Board.
[6] Tech Tip: OBD II Engine Diagnostics, Larry Carley, Import Car, Dec. 18, 2008.
[7] On-Board Diagnostics (OBD) II; Frequently Asked Questions, Ohio EPA website.
[8] OBD-II (Check Engine Light) Trouble Codes, website.
[9] Torque Android App Review & OBDII Bluetooth Adapter, YouTube, Jan. 8, 2011.
[10] Saturday Mechanic: How to Use an OBD-II Scan Tool, YouTube, May 18, 2012.