The first thing to do is always to have the diagnostic fault codes read and recorded. The people at most auto parts stores will do that for you for free. Those codes never say to replace parts or that they're bad. They will indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a part is referenced in a code, it is only the cause of that code about half of the time. You must eliminate wiring and connector problems first.
There's well over 2,000 potential fault codes. About half of them relate to things that could adversely affect emissions, and those are the codes that must turn the Check Engine light on. A lot of those problems that can be detected have nothing to do with the way the engine is running.
The last thing you want to do is disconnect the battery. That will erase the fault codes from memory and you'll lose that valuable information. The goal is to KEEP those codes in memory until they're read. Even then, your mechanic will erase them when the repairs are completed. When a problem is detected and a fault code is set, disconnecting the battery will erase the code(s), but doing that can hardly be expected to fix a mechanical problem. Once the codes are recorded, most mechanics will erase them, then drive the car to see which ones set again. Hard defects will cause their related codes to set again immediately. It's the intermittent ones that drive us nuts. Some require a very specific set of conditions to be detected, or the problem has to occur a specific number of times within a given amount of time to set. In the meantime, you can have engine performance issues with a fault code that hasn't set yet once it was erased. In that case the mechanic has no idea where to start looking. He will have no choice but to hand you a bill, then send you on your way with instructions to come back after the Engine Computer detects the problem again. This is why it is far better to read the codes right away, not to erase them.
Thursday, March 5th, 2015 AT 9:15 PM