All cars manufactured since 1996 and sold in the U.S. have OBD2 system. By accessing to the system, technicians can get Diagnostic Trouble Codes (DTC) that help technicians repair a car greatly. But some DTCs definitions are so vague. You need some further research to fix a problem.

For example, "P0303 cylinder 3 misfire detected". Most of vehicle's computers detect a abnormal change of engine speed (RPM) to detect misfires. Depending where the crankshaft position is when misfiring, the computer shows you even which cylinder cause a misfire. Fixing this problem is relatively easy. Suspected possible causes are #3 spark plug, #3 spark plug wire, #3 fuel injector, #3 fuel injector circuit or #3 cylinder of engine itself in the worst scenario. Technicians narrow them down by further diagnostics.

"P0300 ramdom misfire or multiple cylinder misfre detected". Technician often have a problem with this code. The definition of the code is that misfires occurred on many cylinders. Since misfires happened on many cylinders the computer doesn't even bother to show you which cylinder misfires. Sometimes this code shows up with other codes. So technicians can diagnose other codes first and that would fix all problems. Sometimes it shows up alone. Although it says a car had misfires, a car runs smoothly without shaking. You might want to consider it as a false code.

Today's OBD2 systems are advanced and sensitive to vehicle's abnormal behaviors. This sensitivity could cause some false code to set. For example, driving on a very rough road can mimic continuously sudden changes of engine speed caused by misfires. Some newer vehicles have feature of reducing misfire sensitivity on a rough road.

And some vehicles have different way to detect misfires. While most of vehicles monitor the engine speed to detect misfires, some vehicles monitor the firing voltage of each spark plug. No voltage means no spark. Higher voltage means too much resistance on spark plug, large gap. Lower voltage means exsiting unwanted resistance between spark plug and ignition coil.

False codes are temporary. Clear the code and it won't come back. This is only one example of false codes. There are many of them giving technicians a hard time. They always need to update information, understand a new technology as well as refer to Technical Service Bulletin. And car owners may need to see what's going on with you car, too. If your car have obvious lack of performance you should take it to an auto repair shop right away, but if your car runs normally and the only problem is check engine light on in an instrument panel, you get the code with scan tool, take a note and erase it. And then see if the code is coming back. It'll save you precious time and money paid for unnecessary repair to a dishonest mechanic shop.