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posted: 8/28/2011 12:01 AM

Under the hood: Checking a code is not diagnosis

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Q. I recently had a "check engine" light come on in my Toyota. I took it over to one of the parts stores and they diagnosed the car for free. They said it had what they called an evap code stored in the computer. They suggested I try a new gas cap, which I did, but the light came back on. When I took it to the shop they said they would diagnose the car and it could be as much as $100 or so.

How can it be free at one place and $100-plus at another? I need to get this resolved because I am due for my emissions test.

A. This is a good question and one that comes up a lot. What you had done at the parts store was not a diagnosis. They merely read the codes that were stored in the computer. If they called that a diagnosis, they should not have.

The suggestion to try a gas cap was an educated guess and actually not a bad place to start. If it is the same code that is back, you will have to spend some money for a good diagnostic technician to track down the real source of the problem.

The code merely points you in the direction of the fault, including which system of the engine has a problem. In this case we are talking about the evaporative system. This has to do with controlling the fuel vapors from escaping into the atmosphere.

This fault can be a challenging one to track down because there are several solenoids, valves, hoses and a canister that could be at fault. It also includes the gas tank and the fill tube. Every one of those has to be tested or cycled to be sure it is working properly. The hoses need to be leak-tested because it is a sealed system and the onboard computer is testing the system to make sure it remains sealed. This can require a good bit of time to work through and that is what you are paying for. You certainly don't want to buy a $300 or $400 canister unless you are sure that is what you need.

When the "check engine" light comes on, there could be one of many codes or multiple codes stored. Some of the codes are a little more cut-and-dried than others, but in most cases just having the code is not enough and some testing will need to be done to be sure the proper repair is recommended.

Guessing with a gas cap is one thing, but guessing with a several hundred dollar part is quite another. My suggestion is to pay your shop to diagnose and repair the vehicle. You can then head off to the emissions lane with confidence.

Q. I am curious why I see brakes advertised for these really cheap prices like $59.95, but in reality it ends up costing several hundred dollars to get a good brake job. Is there that big of a difference in brakes?

A. Yes there can be a big difference on several fronts. First, an ad like that is usually talking about the cheapest brake pads that you can get and they most likely will not be made out of the right material to fit your car. Most cars today use either a semi-metallic brake pad or a ceramic brake pad, and you really need to put the right one on for the car to stop as it was designed.

Second, they are not including the price for new brake rotors, which in most cases will be needed. Occasionally the brake rotors can be resurfaced and that is generally extra, too, so read the fine print.

Third, prices vary quite a bit between makes and models so I think it is difficult to put out a menu price that is going to cover all the cars and trucks on the road.

Finally, we are talking about brakes here, this is what stops your car. Do you really want the cheapest thing you can find protecting you and your family? Think about it.

• Douglas Automotive is at 312 S. Hager Ave., Barrington, (847) 381-0454, and 7218B Virginia Road, Crystal Lake, (815) 356-0440. For information, visit Send questions to