The check engine light usually means there is something wrong with your car engine, intake, exhaust or even the ECU. Being a generic warning indicator, it can be difficult to tell which part exactly is giving you problems with the amount of electronics in the car nowadays. Everything from your gas cap not clicking the right number of times to something such as the Idle Air Control Valve can make this light come on. The following are a few examples/considerations as to why the check light will come on.
The last modification done is the culprit. Usually, the last modification done to the car can be the culprit. For instance, change of air filter from drop in filters to open pod might indicate a leakage in the connecting tubes. The exhaust manifold, also known as the extractor, links to the down-pipe, which has O2 sensors to before and after the catalytic converters to detect any problems with it.
Any tampering with the converter might trigger off the check light, too. Valves or pistons that are overstressed and cracked, leading to leakages will also trigger the check light. This usually happens to drivers who rev their vehicles too hard, or who often reach the red line, which is also known as dragging the engine.
Another reason could be a weak battery, but in this case, the check battery light will light up too, and it will probably be impossible to start the engine. Engines that are overheated due to a faulty radiator might be a problem too. The coolant could have leaked off due to a rusty radiator.
Use On-Board Diagnostic (OBD). One good way of knowing what problems the car is suffering from is to get hold of an OBD II scanner, which is meant for newer cars. This scanner is connected to a terminal underneath the steering wheel, allowing it to pick up the codes that the computer is relaying that has caused the check engine light to come on. Some of the codes that are given will either give a general idea of where the issue is, or it could give the exact problem.
Once you have this code, you can then determine where the problem originates, making repair much easier rather than just wondering why. Without it, there is no way of really knowing why the light is on, unless you are a mechanic with the complete know-how in diagnosing an issue. The last thing you want to do is get all worried because the light came on when it was such an easy fix.
Once you've addressed the exact cause of the "check engine" light, but it still comes back on, this will mean that the issue was never properly fixed or there is an additional problem elsewhere. If this happens, be sure to return to repair shop that initially fixed it, because if it were caused by something they did, they would have to fix it. Also, before doing anything to your car, make sure to follow the provisions of the warranty as it may become void if you do fixes outside of the official or accredited service centers.