How to Know if theres a Defect in your Engine

When the check engine lights up.Check Engine - Located at front of the driver seats.Like Odometer, RPM, Tachometer and the Check engine lights.
Today’s cars are run by efficient computerized systems that constantly monitor performance. When a malfunction is detected, the check engine light (CEL) will illuminate in the dash to let the driver know a problem has been detected. In some cases the light might go off by itself, then come back on later. In other cases the check engine light will come on and remain on. Fortunately, when the check engine light is triggered, the engine control unit (ECU) will log one or more codes that can help point mechanics toward the source of the problem.
Engines encompass a number of systems that work together to provide excellent performance under normal circumstances. The vast majority of engines on the road today are powered by electronic fuel injection (EFI). The EFI system employs a number of valves and sensors to ensure the correct amount of air and fuel are fed to the piston chambers to create the right combustion mixture. Timing of the spark is also controlled electronically. If any one of these systems fails, this can trigger the check engine light. 
Another event that is often responsible for tripping the check engine light is a vacuum leak. In this case a hose or manifold might be loose or cracked. A visual inspection can help, but in some cases a vacuum leak can be very difficult to locate. Some repair shops offer a smoke test for about $60 US Dollars. A smoke generator made for this purpose is used to fill the air intake system and hoses are closely monitored for escaping smoke. 
 Other vacuum leaks that can trip the check engine light include a sticky exhaust gas recirculation (EGR) valve, or more seriously, a blown head gasket. Pinched or deteriorated fuel injector o-rings might also be the culprit. Dirty fuel injectors or a faulty oxygen (O2) sensor might also be to blame. 
Because there are so many possible reasons for the check engine light to come on, pulling the codes for the event can be helpful when the problem is not immediately evident. Even if the check engine light is only on a short time, a code will be stored in the ECU for later retrieval. 
US vehicles manufactured prior to 1996 feature a diagnostic module in the engine compartment by the main fuse box. Often a label is affixed to the underside of the hood explaining which terminals to access. Using a simple metal tool (even a paper clip) to connect the two terminals will cause the check engine light to flash in the dash when the key is in the accessory position. By reading the number of flashes one can determine which code was set. Manufacturer’s code guides, available online, will reveal which system(s) tripped the check engine light.
US vehicles manufactured after 1996 are required to feature a Molex-style connector under the dash, normally by the driver’s side. These connectors can interface with a dongle-to-USB cable to plug into a laptop running auto diagnostic software. The software not only reveals the code(s), but corresponding possible cause(s), and can perform many other diagnostic functions.
A check engine light doesn’t always mean a big problem or an expensive repair is ahead. However, driving a car with the check engine light on could mean that you are doing further damage to the engine, depending on the problem. It’s always best to get the vehicle checked at your earliest convenience.