Will it Works with My Vehicle?

The most important aspect we tool buyers have to consider is that the tool we purchase must work with the troubling vehicles models. It's true that OBDII is a standard, but in the market there are three different types of hardware communications used by OBDII vehicles. Some tools support all three and some are manufacturer specific. If you're a diehard Chevy customer, you may be able to save a few dollars by choosing a manufacturer specific tool.

Supported parameters
Not all scantools are the same,actually, some ones aren't even close. As part of the OBDII standard, the US Environmental Protection Agency mandated that on all OBDII vehicles it supports a basic set of  reading-related emissions. The SAE specification J1979 defines these legislated parameters. Many low-end tools only support these emissions related readings, giving you access to only a dozen or so truly useful parameters. While these give you some basic vehicle information, they are just a small set of the vehicle information available through the OBDII port. When shopping for a scantool, be sure to find out if it supports just the legislated parameters or if it also includes the enhanced (sometimes called proprietary) diagnostic.

It can be updated by CD or via Internet or not upgradeable?
Each year new vehicle models are released or existing models revised from vehicle manufacturers. In order to fully support the new vehicles, a scan tool must typically be upgraded. Professional quality scantools are upgradeable, although often at a price of $500 or more per upgrade. Most lower end handheld scantools are not upgradeable. Check what upgrades will cost before committing to a tool.

Scan Tools Built in Help about Factory Service Information
The Factory Service Manual have to be owned by anyone working on his or her own vehicle. But the scantool itself may be able to provide some of that necessary information. When a DTC is set, does the tool display the DTC number or give the full definition? A tool that displays the full definition will save a lot of time and frustration. Does the tool offer any information on typical readings to explain what the reading is? A simple sentence or two of explanation can save a lot of trips back and forth to the shop manual.

Data logging or storage
A 100% way to park your car in a ditch is to try and watch a scantool display while doing a roadtest. It's very necessary that a tool stores data to allow safe viewing back at the garage.