I am an auto mechanic and have seen many times where people have driven their vehicle with a check engine light on for a good year before bringing the vehicle into a shop. Many people postpone having it looked at because they are afraid it is going to cost a lot of money. Many times the fix was easy as just putting a hose back on, which cost only 10 minutes worth of work and no parts to replace. The hose that was off cost the person a loss of 3 miles to the gallon. I am going to give you some information on things you can do yourself to fix your vehicle and to get rid of that annoying light.
You have a couple of options in ways to check for codes. All vehicles that are 1996 or newer will have the same style connector which is known as a DLC (data link connector.) You can buy a cheap code scanner from many places such as Wal-Mart or a large auto chain store, the price will usually be around a little over $100 for a decent one or a very basic one for about $60. If you do not want to buy a code scanner most large chain auto parts stores will scan codes for free, the only disadvantage to this is it will take a little extra time to clear the codes or you have to go back to the store to have the code cleared. I will explain code clearing later.
Once you have found out what code you have you can either buy a repair manual to diagnose the code you have, or use a site like justanswer.com and tell the tech what code you have and let them assist you in solving the issue. A lot of times the parts store that has scanned the code will give you some guidance on what needs to replaced. You can also try searching the web for the code, or join an automotive forum that is devoted to your type of vehicle. Most automotive forums are free to join, and you will find some very smart people who have lots of information to help you out.
I am going to help you with a few codes that I have seen throughout the years. If you have a code that is P0300 through P0316 these are due to a misfire. Most of these codes will explain what cylinder it is on, and if you have anywhere from 80,000 miles to 100,000 plus miles it is most likely due to needing a tune up. Tune ups are due in most cases at 100,000 miles, although I have seen many cheap model car companies who recommend to have the spark plugs replaced at 30,000 miles. A tune up generally consists of an air filter, PCV valve, fuel filter, spark plugs and wires (which if you have coil on plug you would want to replace the coil on plug boots.) If you have a vehicle that has coil on plugs and you have a misfire code that is related to one cylinder you will want to swap the misfiring cylinder coil with another cylinder and see if the misfire changes to the cylinder you swapped with, if it has changed you will need to replace that coil as it is bad. You will also need to check the spark plug to make sure it is not fouled out due to the misfire.
Another common code (mostly on Ford vehicles) is P0401. This code is an emission related code which is usually set by a bad sensor that reads the EGR valve (exhaust gas recirculation valve). If you have a vacuum pump you will want to put it onto the top of the EGR valve, which you will need to remove the vacuum line on top of this valve. Once you have the pump on the top, you will want to start the car and apply vacuum to this pump. If the vehicle dies or the idle drops or engine starts missing this indicates that the EGR is working correctly and not plugged. If you have determined the system is not plugged you will want to replace the DPFE sensor, which is also known as an EGR sensor.
Once you have fixed the issue you can either use your code scanner to clear the codes or go the the parts store who checked your codes and have them clear the codes. You can also disconnect the battery and wait about 15 minutes then reinstall, start the vehicle and the light should be off. If you have replaced a sensor that is used for the computer to read how much fuel to give the engine you will need to reset the KAM (keep alive memory.) The KAM needs to be reset so the computer can learn the new fuel strategy from the sensor being replaced. To reset the KAM you can use a computer that resets the KAM or just disconnect the battery and turn the headlights on. What happens when you turn the headlights on (even with no battery) this will discharge any power stored in the computer so the KAM will lose all power and reset.
If you are looking to save fuel you would be surprised on how much fuel economy you could be losing from that light. A partially plugged fuel filter / air filter can lose up to 15%. If your vehicle is misfiring you could also be losing quite a bit of fuel as the engine has to work harder due to that misfire issue. The best way to save fuel is to have a perfectly running vehicle, if the sensors read incorrectly the computer will compensate the fuel to prevent a engine failure. Which in most cases the vehicle will try to richen the the engine which means more fuel will try to be burnt.
