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Modern Vehicle Computing: On-Board Diagnostics

Scan Gauge II
Jay Kopycinski | Writer
Posted November 1, 2011

On-Board Diagnostics & How We Can Use Them

There was a time when engines had very simple means of controlling the fuel and spark activities needed to turn combustible gasoline into power to move our vehicles. You typically had a carburetor that metered air/fuel flow into the engine. Along with this, spark to the cylinders was often controlled by an ignition system with mechanical distributor that could advance ignition timing as rpm increased via intake vacuum and/or centrifugal weights on the distributor.

In the late 1960s and early 1970s, car manufacturers started to implement basic electronic controls for fuel and spark management. However, the early systems were manufacturer specific and/or used a proprietary configuration. Fast forward to about 1988 when the California Air Resources Board (CARB) mandated some form of on-board diagnostics, and OBD (or now called OBD-I) was born. On-Board Diagnostics (OBD) is a system by which automotive drivetrain management is monitored and controlled. It also has the ability to report fault conditions to the engine control module. OBD-I was a start, but there was not yet a standardization and systems varied from manufacturer to manufacturer.

Some years later, CARB issued an OBD-II specification and mandated that 1996 vehicles use a standardized diagnostic connector and data output format. Implementation of OBD-II meant that vehicle emission performance could be checked by plugging into the vehicle diagnostic connector as opposed to having to run the vehicle through a loaded dyno roller test.

Current OBD-II systems have expanded to allow the monitoring of numerous powertrain characteristics. The list of parameters is long (several hundred possibilities) but can include:

  • Engine Coolant Temperature

  • Throttle Position Voltage

  • Engine RPM

  • Oxygen Sensor Voltage

  • Vehicle Speed Sensor

  • Intake Air Temperature

  • Ignition Timing

  • Fuel Pressure

  • Mass Air Flow Sensor

  • Engine Fuel Rate

  • Evaporative System Status

  • Misfire Monitoring

  • Transmission Gear

  • Air Conditioning On/Off

  • EGR System Status

As you drive an OBD-II–equipped vehicle, it is constantly using data acquired from the various sensors to run the fuel injection system. Ambient air temperature, engine coolant temperature, throttle position, and intake air volume are all used to adjust the electrical pulse width (time injector is turned on) of fuel injectors to add more or less fuel as calculated by a data map stored in the engine computer. This estimate is further adjusted by reading the exhaust composition via oxygen sensors before and after catalytic converters.

Engine control now becomes far more sophisticated and can be optimized for better fuel economy and fewer tailpipe emissions. Additionally, the computer revises its fuel maps as it learns the specific driving patterns and performance of the specific engine it is controlling.

Code Reading

As we mentioned previously, OBD have the ability to monitor drivetrain systems for failures that might occur and have some ability to report faults back to the computer. In the early days of vehicle computers, we might short two diagnostic electrical terminals and observe a flashing pattern of a “check engine” light to decipher a trouble code to aid in troubleshooting a problem. OBD reports sensor and fault data through its diagnostic connector, and a code scanner is used to report a text message to the observer.

When the engine is first started, it operates in what is referred to as “open loop” condition. In this case, the electronic controller operates the fuel injectors based on a stored data table and its best estimate of engine conditions, using some sensor data. After a brief warmup, the system switches over to “closed loop” mode. In this condition, the full array of sensors is active and provides calibrated feedback to the controller to optimize the fuel and timing settings on a continuous basis while the engine is running.

For a code example, engine control modules can detect a short or open circuit condition at a fuel injector and report the injector failure to the microprocessor in the module. From here, a technician can read the code with a scanner and work to locate the faulty component. We found useful code descriptions at


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