Why is my Check Engine Light on
Your car's check engine light can come on for a variety of reasons. The most extreme cause is engine failure of some kind that would require emergency attention. In such cases, most check engine light will flash repeatedly. If your check engine light ever begins blinking, stop your car immediately, as only extreme engine issues will cause the light to blink. In other situations your car may not start, possibly begin running roughly while driving, or even stall entirely. Under these circumstances the cause for the light will simply require a mechanical inspection to determine the cause of your problem. However, it may come on under normal operation, and these are the situations that we need to understand.
If you start your car and the check engine light stays on, or if you are driving and your light comes on without any apparent reason, the problem is generally electrical in nature. One of hundreds of sensors can fail, or send mixed signals to your car's computer that can set a check engine light on. One of the easiest fixes can be simply tightening the gas cap as many cars light can be turned on by a sensor that senses inproper pressure in the fuel tank. If that's the case, your light will usually turn off, and you can go about your day. In almost all other cases you will have to "scan" your car with an automotive code reader. Before we get to the fix however, let's focus on what's happening when the light comes on. Did you hit a pothole in the road? Did you slow down or accellerate erratically? If you focus on what is happening with the car when the light comes on, your information can help a qualified mechanic with his inspection.
Code readers can differ in the amout of information you can obtain, so it's important that you understand the difference. The code reading computer that a dealership will have offers a much larger span of information than the cheaper models employed by most general repair shops, and parts stores. Of course the dealership will also charge a great deal more for their services. On the other hand, a repair shop's scan can still pin-point many issues, and generally cost less. The key factors in the quality of a repair shops scan are: 1.) The experience and knowledge of the operator. 2.) The quality of the scan tool used. 3.) The complexity of the problem at hand. If you find a good scan technician keep them!
Of course the do-it-yourselfer can always buy a scan tool at a parts store. Some parts stores will even scan it for you, or rent you a scan tool. In a scanner you must know if your vehicle is an OBD1(95-older), or and OBD2 (95-newer). Also, check your vehicles plug connection for the scanner (usually under the drivers dash) to make sure you get the right cable to scan your vehicle. You can pick up a reasonable scanner for $100-$250 that will give you the information you need. The more you spend the more information it will generally read. The instruction manual should help explain the code if the scanner itself only gives you a numeric code.
A common sensor to fail is the oxygen sensor in your exhaust. It tells the computer how much air is being pushed through the exhaust so that the computer can adjust the fuel:air ratio. When it fails, the check engine light will come on and your performance may be affected. Your gas milage will certainly decrease due to the false readings. Other than that your car will run generally as normal. Any part of the firing system can fail and set the light on as well. A spark plug, plug wire, coil pack, cap, or rotor can all set off a check engine light. Of course, low oil or coolant can set the light on as well, however, most vehicles will have gauges that will warn you of that long before your check engine light comes on.
In reality most times a check engine light comes on it is not an emergency, and if it is, you should be able to assess the situation your self. When attention is needed it is almost always caused by an electrical part, or sensor. The decision is then up to you as far as how you choose to deal with it. If your car still runs well, try the cheaper scan and see if it picks up something simple. If it dosen't seem the same when the light is on have a dealership give it a full scan and get it right the first time. Nobody can simply look at a car and tell you why the check engine light is on, but with a few simple tests, and the right scan you can be back on the road in no time
