There's over a thousand potential diagnostic fault codes but I can offer a couple of observations. First of all, fault codes never say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis. Reading the fault code is the first step in the diagnosis. The second step is to perform electrical checks, typically at the sensor, or at the computer. Once any wiring or connector problems are eliminated, THAT is when the mechanic comes up with the suggested solution which could be to replace the sensor. If you were given the recommendation of replacing a sensor based solely on the fault code, it's time for a second opinion. The exception would be if the sensor in question has a known very high failure rate or if an experienced mechanic recognizes a familiar pattern such as an engine hesitation or some other clue that is caused by that sensor. In the absence of that experience, if a sensor is replaced simply because it is referred to in a fault code, the problem will persist about half of the time.
The second thing here is to know exactly which code was set. There is no such thing as a "smoke emission" sensor. This is an all-too-common example of miscommunication. Mechanics do a real good job of communicating with other mechanics but a very poor job with car owners. Add in a person behind the counter who is supposed to translate what the mechanic found into something you can understand, and there is going to be something lost in the message. Most people behind the counter never were a mechanic and they often know little more about cars than you do. This is why we need the exact code number. With that, we can suggest a course of action for you to diagnose the cause of the problem or to help you make an informed decision with your mechanic.
The fact that the Check Engine light is off each time you start the engine suggests the problem is relatively minor as far as emissions is concerned, but not necessarily the cost. With less-severe problems the light will turn off while you're driving if the problem stops occurring. About half of the codes refer to things that will not adversely affect emissions, and those will not turn the light on at all.
Also, since the light is off at times, that means the problem is either very intermittent and not a constant defect, or it doesn't show up until the Engine Computer runs certain self-tests to check circuits for proper operation. You can help in the diagnosis if you can recognize a pattern to the operation of the Check Engine light. It may always turn on only during hard acceleration, or it might always be close to the same number of minutes or miles after starting the engine. Anything you observe might help the mechanic speed up the diagnosis.
Monday, September 30th, 2013 AT 11:34 PM