QUESTION: The information you supplied about going to an auto parts store to get a "check engine" light diagnosed is wrong. I tried that a few months ago and was told by AutoZone that our illustrious California legislature has forbidden parts stores from performing that service unless they also will repair the problem.

—Ron Johnstone

ANSWER: Ron, you are correct that in some areas — California and Hawaii among them — certain auto parts stores are no longer offering to retrieve diagnostic trouble codes for customers in the parking lot. The reason is this service lies in a grey area between offering a helpful free service and, on the other hand, diagnosing and repairing vehicles without an automotive repair dealer license.

Additionally, in California, emission-related repairs — all "check engine" light issues — can only be performed by a licensed smog technician or the vehicle owner. It’s my understanding that in California, rather than a law, there is an agreement between the state Bureau of Automotive Repair and the big chain parts stores to steer clear of this area. Smaller chains and independents seem to be under the radar thus far.

Free "check engine" light diagnosis is a controversial topic. Consumers can certainly benefit from becoming aware of the severity of a "check engine" light issue, such as whether the engine will quit or become damaged, or whether a repair can be put off. Repair shops likely cringe at the thought that a non-technician may be alluding to or recommending a certain part be replaced or service be performed, based solely on a diagnostic trouble code being present.

Why is it so difficult to be sure of a certain repair or failed part when the "check engine" light comes on? A modern vehicle’s onboard diagnostic system is very smart, but almost all identified problems require a human to test additional things before the exact cause of the fault can be found.

Some diagnostic trouble codes, or DTCs, point fairly clearly at a failed part. Take DTC P0135: oxygen sensor heater circuit malfunction: bank 1, sensor 1. In most cases, it will be fixed by replacement of the bank 1, front oxygen sensor. It’s also possible a wiring fault or other issue could be the cause. Throwing an O2 sensor at the car, in this case, may fix the problem perhaps 9 out of 10 times.

In another case, DTC P1259 indicates a variable valve timing/lift problem on a Honda, and possibly an expensive part to be renewed. Skilled diagnosis leads to a pleasant conclusion: a simple oil change, using the correct viscosity oil, makes everything right again. Or a Volkswagen Jetta Turbo has a half-dozen or more DTCs at one time — imagine renewing expensive parts for each code on this one — and the single cause turns out to be a failed PCV diaphragm. This simple mechanical fault causes such odd-ball symptoms that onboard diagnostics simply can’t figure it out.

The bottom line is that retrieving a trouble code is the first of many needed steps when diagnosing an illuminated "check engine" lamp. Leapfrogging to a conclusion works occasionally, but in many cases leads to unnecessary parts replacement or work performed. A vehicle owner with DTC and proper tools in hand should not be discouraged from attempting repairs, as long as he or she finds and follows available published procedures.