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Article posted: 9/22/2013 12:10 AM

Computer code is not full diagnosis

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By Doug McAllister

Q. When the "check engine" light went on in my car, I took it to the parts store and they gave me the fail code for the evaporative system. When I took the car to the repair shop to get it fixed, they said they would need to charge for diagnostics to find out what needed repair. Doesn't the code tell you what needs repair?

A. The short answer is no it doesn't.


The major parts retailers have done all of us a disservice with their so-called free check engine light "diagnosis" because it leads you to believe that is the whole answer. In some rare cases the code will point directly to a part that has failed, but more often than not there is some work to be done in order to get the whole answer.

In the case of your evaporative code, the problem could be as simple as a loose gas cap or a cracked vacuum hose, or as complicated as a rusted gas fill tube or a canister purge valve. You just don't know until you test each one of these components to find the culprit.

A good diagnostic technician, with the proper tools, will get to the source of the problem fairly quickly and minimize your parts cost. We had a Jaguar in the shop the other day that had a whole page full of codes. When the tech started working through the system, he discovered a bad ground cable. Once this was cleaned, replaced and the computer codes cleared, the problem was resolved. How many unnecessary parts would have been sold had this computer fail code not been diagnosed properly?

We have had people bring us the part that a parts store told them he or she needed, only to find out that was not the problem at all.

Really the answer to the question is to find a shop in which you have trust and confidence, and it will steer you to the most cost-effective repair when that pesky check engine light comes on!

• Douglas Automotive is at 312 S. Hager Ave., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit Send questions to

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