A common misconception yet, even on the part of some mechanics, is the Check Engine light has to be on to read fault codes. As Steve W. Said, that is absolutely not true. There are well over 2,000 potential fault codes that can be set. Only about half of them refer to things that could adversely affect emissions. Those are the codes that turn on the Check Engine light. The light does not turn on in response to over half of the codes.
Without those diagnostic fault codes, it could take a mechanic weeks to figure out the cause of some problems because he would have no idea in which of the dozens of circuits to start looking. When the problem is intermittent, the only testing that has any value is that which is done while the problem is acting up. At all other times there is no problem which means there is no defect to be found. The fault codes are stored in memory so the mechanic can read them later and know which circuit or system needs further diagnosis.
As long as I'm adding to my wondrous story, I should add that fault codes never ever say to replace parts or that one is bad. They only indicate the circuit or system that needs that further diagnosis, or the unacceptable operating condition. If you'd read through the list of fault codes, you'd see that no part is even mentioned in the majority of them. The people at many auto parts stores will read codes in the Engine Computer for you for free, but they will usually try to sell you a sensor or some other part because selling parts is what they understand.
Your mechanic is right to not want throw random parts at a problem. There are times when we have no other choice, but you have to understand that you'll need to drive the car to see if the problem acts up again, then you are expected to have too return to the shop again. A better alternative that can help is to drive the car with a "flight recorder". That usually involves connecting an expensive scanner or some other piece of diagnostic equipment, and the shop may not be willing to let that out of their sight. Scanners with a "record" feature work best when the problem acts up often enough that the mechanic can make the recording on a typical test drive. New-car dealerships often have a smaller version that can be connected for the owner to take with them for days at a time. You simply press the "record" button when the problem occurs. A few seconds of sensor data is recorded for the mechanic to play back slowly later, then he can watch what changed. Because that data passes through the tool's memory, the recording actually starts a few seconds before the button was pressed.
It is also helpful to understand how some fault codes are set. Many sensors are fed with 5.0 volts and ground, (0.0 volts), then they develop a signal voltage within that range. The throttle position sensor has mechanical stops the limit its range of travel to 0.5 to 4.5 volts. If a wiring or sensor problem causes the signal voltage to fall outside that range, that is when the computer sets the fault code. Other sensors are limited electronically to that same acceptable range. Some sensors, particularly electronic sensors, can develop a wrong signal voltage, but as long as it stays within the acceptable range, no fault code will be set. That is when the mechanic has to try to figure out what doesn't look right. Some sensors have very little effect on engine performance, but some are critical and their signal voltages must be very precise. On most cars other than Chrysler products, the main sensor for fuel metering calculations is the mass air flow sensor. It sits in the fresh air tube, and a loose hose clamp or even a small crack in that tube can cause a noticeable engine performance problem.
Having multiple computers on our cars today makes finding which circuit to diagnose much easier. The problem is we have a lot more electrical problems than ever before because of those computers.
Wednesday, January 18th, 2017 AT 3:26 PM