Diagnostics on Tap: Utilizing OBDII for Vehicle Diagnostics

In the first part of this article, we explored the various sensors that gather and transmit information to the ECU. After the computer processes all of these inputs, what happens next? Everyone has no doubt experienced the ominous "Check Engine Light" at some point in their life. Let's take a look at how modern automotive electronics have made it possible to quickly and easily to diagnose problems using codes and scan tools.


In the early days of automobiles, troubleshooting was limited to the five senses and a few gauges. Advancements in modern electronics have changed the way cars are maintained and repaired, allowing problems to be easily pinpointed and more quickly diagnosed. On-board diagnostic (OBD) systems enable the use of computerized scan tools to communicate with a vehicle’s ECU and are slowly but surely removing humans from the loop.


The first generation of on-board diagnostics (OBDI) was adopted by the California Air Resources Board (C.A.R.B.) in 1985 and implemented in 1988 and subsequent model-year vehicles. These systems were able to perform limited functionality and circuit continuity checks of certain components.


In 1989, C.A.R.B. approved the next generation OBDII system, which was mandatory on all cars built in 1996 and later. This new technology increased the scope of monitored components and provided more specific performance criteria for determining problems. To find out if your car is OBDII, refer to the Vehicle Emission Control Information (VECI) label, typically located in your car’s engine compartment.

The OBDII system monitors almost all emission-related components and malfunctions that can lead to increased emissions, including engine misfires, catalytic converter(s), oxygen sensor(s) and the exhaust gas recirculation (EGR) system.

Where is my ODBII port?

OBDII regulations effectively standardized the entire system, requiring manufacturers to use the same data link connector for all vehicles to ensure proper communication with a variety of scan tools. The OBDII port must be located within three feet of the driver and can usually be accessed without any tools; under the dash (above the pedals) or behind the ashtray are common locations for the port.

Interpreting a DTC

Diagnostic Trouble Codes (DTCs) are reported in a universal format so that all technicians and scanners are able to read and understand them, regardless of the application. A large number of codes are generic, while enhanced (vehicle-specific) DTCs vary depending on the application.

All OBDII codes are five characters long and can be easily interpreted.

Universal scan tools can be purchased at any major auto parts store at relatively little cost; make-specific models are more expensive.

The Snap-On Ethos® has the ability to read proprietary, vehicle-specific codes for Chrysler, Ford & GM, with extended support for the major makes of Asian imports.


While pulling a DTC might inform you of a condition (such as running lean or misfiring,) it will not always tell you exactly what is causing it. What it will do is help you zero-in on the problem and more effectively diagnose it. Datalogging refers to the ECU's ability to record and track engine performance over time, including the values for all sensors and any codes that were thrown. Scan tools can then access this information to paint an accurate picture of your vehicle's operating characteristics; datalogging is absolutely critical for performance tuning.


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