I’m thinking about buying a code scanner that’s featured at my auto parts store. My question is, how well do these $150 scanners work? My mechanic told me once his professional-grade scanner cost $2,000 plus cartridges. What’s the difference?

Mark Porter,

Dublin, Calif.

Automotive scan tools have come a long way since their assembly-line origin in the early 1980s. Cars were checked as they rolled off the line to be sure their primitive engine control system was properly functioning.

Handheld scan tools soon followed, allowing repair technicians to retrieve trouble codes, and observe sensor and output data. As vehicle technology increased, scan tools became able to direct functional component and system tests and record road test data. Other vehicle systems such as transmission, brakes, cruise control, air conditioning and body electronics next became scan tool accessible.

Scan tools used in repair shops might be manufacturer-specific units capable of very advanced functions or after-market tools offering multimake compatibility, with modest to good capabilities.

Purchasing and maintaining the myriad needed adapters and program cartridges was and is expensive and problematic. Since 1996 all vehicles sold in the United States must be capable of communicating basic emission system information to a generic scan tool through a standardized data link connector. This OBD-II (on-board diagnostics generation II) generic format allowed a flood of inexpensive PDA and laptop-based scan tools, as well as standalone and even build-it-yourself units, to trickle down to the home mechanic.

Are these inexpensive OBD-II generic scan tools useful? Yes, but one must be aware of their limitations. These units can retrieve and clear emissions-related trouble codes; display a modest list of engine/emission system data; indicate system test status; and replay a snapshot of operating conditions whenever a system fault develops. Unfortunately, the data list and lack of advanced features falls far short of a manufacturer-specific or pro-grade-specific generic unit, and other vehicle systems aren’t accessible.

In the right hands, an OBD-II generic scanner can provide the necessary diagnostic information to repair about 50 percent to 70 percent of typical “check engine” light issues. One also must keep in mind that obtaining a trouble code is only the beginning of the diagnostic process. For example, a code might indicate the throttle position sensor circuit voltage is excessive.

There could be several causes, such as a faulty sensor, a circuit malfunction or even a hallucinogenic computer.

Should you buy one? I’d first take a look at a few shop manual trouble code flow charts and gauge your ability to perform the listed steps. If you can, then owning a basic scan tool could make you the neighborhood hero, as well as providing the satisfaction of tackling and resolving a handful of pro-level problems.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-12-30-04 1545EST