Diagnosis code often only a starting point

Q. I have a 2007 Camry that has a "check engine" warning light on. I took it to one of the parts stores and they told me the code was for an oxygen sensor, which I bought. I installed the sensor and cleared the codes but the light came back on. I'm a little frustrated because now I have spent money and time and still have the problem. Any ideas?

A. First off, you have learned a valuable lesson with a fairly inexpensive part. You cannot make a diagnostic decision based on a code alone. This is why the so-called "free diagnostics" that some of the parts stores offer is a bit misleading.

I like to make the analogy that a code will generally tell you what aisle of the supermarket where you can find an item, but it does not tell you what shelf it is on. It takes a skilled diagnostic technician armed with the code, the right tools and the proper information to get to the bottom of most DTC's (diagnostic trouble codes).

With theses codes, there is research to be done and pinpoint tests to perform to accurately rule out good components and condemn the faulty one. While you have to invest some money into the diagnosis labor, in the end you often save money by replacing or repairing only what's needed as opposed to replacing parts that you don't need - like in your case the oxygen sensor.

At this point the only advice I can give you is to take it to a good shop that you know and trust, tell them you would like to have this problem diagnosed and also tell them what you have already done. The more information you give will save the technician some time and you some money. Good luck and I am sure you will get this resolved quickly.

• Douglas Automotive is at 417 W. Main St., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit douglasautomotive.com. Send questions to underthehood@dailyherald.com.