Scan Tool Is Useful But Has Limitations

Q How useful are the under-$100 scanners you see at the auto supply store? They make it sound like you can fix anything with them.

— Gene Hammond

A: The tools Gene is referring to are OBD-II scan tools and code readers. A scan tool connects to the vehicle's data link connector and allows a look at information received and provided by the engine control computer. A code reader displays only alphanumeric diagnostic trouble codes and possibly a code description for engine or emission problems.

Cars built since 1996 (1997 for light trucks) are required to communicate in a standardized format for emission-related information, but not other vehicle systems. The OBD-II data list is very helpful but brief compared to the much more comprehensive list of engine and vehicle parameters available on a professional-grade scan tool.

An inexpensive OBD-II scanner is a great tool for determining the cause of an illuminated check engine light but has limitations when it comes to fixing the problem. After obtaining a trouble code, one typically goes to the manufacturer's flow chart for that code, using the scan tool and in many cases a digital multimeter or other diagnostic tools to track down the specific fault. If the scan tool is capable of displaying a wide range of sensor readings, or can command component functions, it can reduce the time spent with the other tools.
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The key of using an automotive scan tool, understand you Scan Tool Limitations, its one microprocessor talking to another.

A good example is an Open Coolant Sensor, the computer sees -40 degrees f and it relays it to the scanner. I hoping you know -40 is a very, very, very cold day, and most likely you will not be working on any body’s car. So back to the example; most likely this data or PID, “Parameter Identification Data” is a clue that something is wrong.

So now it’s your job to figure out what’s wrong, that’s when trouble shooting plays a strong role when using a scan tool. You have to understand what you are looking at, and how the computer is interpreting that data.

I will be trying to help you understand some of the data or PID’S you will see on the scan tool.

By the way a -40 degree reading is saying the circuit has an open, either in the sensor it’s self or the wiring.

What are the Scan Tool Limitations, it can not tell you if the sensor is bad or the wiring is broken, it can not tell you if the PCM is bad or the wiring is broken to the computer.
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