When your check engine light comes on, it is important to figure out why and get it repaired as soon as possible. By not getting it repaired, you run the risk of having multiple problems crop up without knowing about them.
Your computer continually runs a self diagnostic check, making sure that the car is running properly. When it sees a problem, the check engine light is signaled to go on. If it is not repaired, the light will remain on and in some instances, the self diagnostics will not be able to go through the other checks beyond that point. If this goes on for several months, when you do bring it in for repairs, your technician may not be able to see ay other issues that may have come up after the initial problem occurred. This can cause some unfortunate circumstances for both you and the technician. The following is the scenario that usually happens.
The technician will access your computer, find the circuit that is showing the failure, test the system, and let you know what the specific problem causing the failure that the vehicles computer is seeing. When the repair is complete, the computer will start its diagnostic check again. When the current issue is passed, the computer continues on its testing. If there is another issue, It will them be able to see it and trigger the check engine light again with the new problem. If you did not inform the repair shop of the extended period of time between the check engine light coming on and you bringing your vehicle in for service, they won't be able to warn you of this possibility. You will be upset that there is more money to be spent to repair the "same" problem, the check engine light coming on.
So, be prepared that there may be more bad news coming if this is your vehicle. Make sure you ask all the right questions and give all the information to the repair facility about what the situation is with the vehicle. This will help to avoid any misunderstandings.