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THE GOODS : Getting the Right Diagnosis on Car Trouble


As cars have become more sophisticated in recent years, the electronic systems that diagnose problems have kept pace.

On the dashboard of newer cars, you will see several indicators that give status reports on air bags, brakes and engine systems, as well as the traditional warnings for low oil pressure and overheating coolant.

The biggest changes in these diagnostics are coming in the so-called check engine light, which can monitor dozens of functions. I regularly get mail from readers complaining of confusing check engine lights.

The "on-board diagnostics generation two" are just being installed on new cars and will be on virtually all 1997 models, auto makers say.

On General Motors cars, for example, the first generation of diagnostics systems monitored about 40 systems for trouble. The second generation will monitor several hundred systems, depending on the car model.

On the older system, the check engine light would come on if an oxygen sensor failed. On the new system, the light will activate if the sensor is not performing up to 100% of the original specifications.

Here's the rub for motorists: In many cases, you might not care if one of these 200 or more systems is not operating perfectly and you certainly might not care to spend half a day or a couple hundred dollars at the dealership to find out what the problem is.

The news isn't all bad, however. There will be two check engine lights. One will say "check engine soon" and another "check engine now." It would be even better in my book if the two lights were to distinguish between problems that would disable or damage an engine versus ones that might only affect fuel mileage or emissions.

On the older systems, it was possible for a mechanically adept person to extract the trouble code from the engine computer. But doing so on the new system requires a dealer or a minimum $100 investment in a hand-held diagnostics instrument.

As a general rule, experts say that if your engine oil pressure and temperature are operating within the normal range, there is no reason to rush to a dealership every time a check engine light activates. Once a check engine light comes on, the trouble code is stored inside the engine computer for 50 cycles of the ignition switch.

If the check engine light persists for several outings, you should have it checked the next time the car is serviced. In the case of the air bag or brake warning lights, they almost always should get immediate attention.


Vartabedian cannot answer mail personally but will attempt to respond in this column to automotive questions of general interest. Write to Your Wheels, 1875 I St. N.W. #1100, Washington, D.C. 20006.

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