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Modern Vehicle Computing: On-Board Diagnostics

Posted in How To on November 1, 2011
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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.

Data from the engine control module can be routed to a diagnostic scan tool plugged into the OBD access port (usually located under the dash but may also be found under the hood for OBD-I).

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

There are conditions that may trigger a fault code in the system, causing the control module to store the code. Depending on the particular fault that occurred and whether it is continually detected, the fault code may set the ‘Check Engine Light’ (CEL) or ‘Malfunction Indicator Light’ (MIL) on the dash. This signals the driver of a persistent fault in the system but often does not stop the vehicle from running. In fact, the MIL may be on but the vehicle may be running normally.

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

Handheld code scanners have become popular and their price has dropped considerably in the last few years, making such tools more practical for the weekend gearhead. With these units you can typically read fault codes, monitor various drivetrain parameters (both in real-time and take snapshots at a chosen time), and clear the fault codes in memory. Once you pull a fault code with a scanner, you can reference the OBD-II code descriptions and start to get some idea as to what the problem is.

OBD-II Drive Cycles

A vehicle with an active fault code will not pass an emissions test. A scanner can be used to clear the code, and if the fault condition is no longer present, the ‘Check Engine Light’ (CEL) or ‘Malfunction Indicator Light’ (MIL) will remain out. Whenever you perform a repair on an OBD-II vehicle, disconnect the battery, or clear the fault codes, the system must run onboard diagnostic tests to determine the state of all the sensors and control modules. Essentially, the system is looking to see a series of actual driving conditions over which to make emissions-related tests.

These drive cycles can vary by manufacturer and model, but can be found online with a little research. Until these drive cycle tests are complete, the ‘readiness monitors’ will not be set and the vehicle cannot be tested for emission compliance. If you’ve done recent work or a code reset, you want to check the monitor status with a scanner to ensure readiness before making a trip to the emissions test station.

Prior to the OBD-II standard, vehicle manufacturers used a range of non-standard or proprietary diagnostic systems and standards. Some code scanners worked only with certain vehicle makes, or a universal scanner might be paired with a number of cable/interface adapters in order to communicate with the various vehicle makes. A single OBD-II compatible scanner can work with any OBD-II compliant vehicle to read the standard parameters and clear fault codes.

OBD Monitoring

Now that we know a bit about what’s running around inside an OBD-II system, how can we use that information to our advantage other than troubleshooting a fault we experience? We took a look at several OBD monitoring tools on the market. These are intended to be placed in the vehicle cabin and can be used for real-time monitoring of powertrain conditions. What used to cost thousands of dollars to get detailed access to OBD parameters is now available in several user-friendly and economical formats.

Each of these units is able to read a basic set of real-time OBD parameters, aid in checking and improving fuel mileage via driving habits, read diagnostic trouble (fault) codes, and clear codes in memory. However, each also has some unique abilities and choice of unit can come down to specific needs for the devices.

One product we tried was the GoPoint Technology GL1 interface. The data cable allows you to connect your Apple iPhone, iPad or iPod Touch to your OBD port. GoPoint includes their GL1 Scan Tool software for use with the interface so you can check trouble codes, clear codes, or monitor OBD engine parameters.

ScanGauge II from Linear Logic is a compact OBD monitoring unit with an LCD screen capable of displaying four monitored values simultaneously in a digital gauge format. In addition to the real-time gauges, it also has a trip computer to monitor mileage and other trip statistics, so it can aid in determining calibrated fuel economy and help you compare the results between different trips. A digital speedometer function can be calibrated to adjust for a change in tire size or gearing.

As with other monitor tools, the ScanGauge II can read diagnostic trouble codes and clear the codes in memory to turn off the MIL. It can also indicate if the readiness monitors are complete in order to make a valid emissions test. For those wanting to dig much deeper into available OBD parameters on their vehicle, ScanGauge II now offers an included feature named XGauge which allows access to the enhanced OBD parameters beyond the basic diagnostics. Linear Logic has a substantial list of enhanced data types on their website and is constantly adding new ones.

With the GoPoint cable connected to our OBD port and to our Apple iPod Touch we had the ability to monitor a good number of engine parameters. Then we tried a $49 program called Dash Command from Palmer Performance Engineering (available from the iTunes App Store). It comes with five dashboards already programmed with virtual gauges, but you can download their DashXL™ dashboard editor that allows you to fully customize your own dashboard design on your home computer.

The graphical interface can monitor fuel economy, log OBD parameter readings, provide a detailed trip computer, and serve as a diagnostic tool for troubleshooting. Dash Command also uses the GPS (iPhone/iPad) and the accelerometer on the host devices to provide real-time race track satellite imagery, an inclinometer showing pitch and roll, plus a skid pad display that can show acceleration & deceleration forces of your vehicle.

Bully Dog offers their Watchdog unit that serves as both as an economy monitor and a performance gauge instrument. In the mode shown here, it can display four monitor parameters on the left and show an enlarged gauge of one of the parameters as well.

The gauges can display up to 15 different parameters (depending on vehicle), has the ability to set temperature and pressure warnings, and can read and erase diagnostic trouble codes.

An interesting feature is the onboard driving coach. In addition to displaying current and trip fuel mileage, the unit can provide visual and audible indicators to show how efficient you are driving. You program all your specific vehicle details so the coach and other features are matched to your engine displacement, vehicle weight, etc. so you can quickly learn what driving behaviors can save you gas money. For lead foot testing, you can also log and store ¼ mile times and similar performance statistics.


Bully Dog
Mesa, AZ 85202
GoPoint Technology
Palmer Performance Engineering

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