This is a great question with a tough answer. The check engine light or service engine soon light monitors anything the engine control module keeps up with. That is a ton of stuff! It is important to get it tested, because many things it monitors can cause bigger problems, if left in disrepair. Somethings are not as critical. For example, a light on due to an air/fuel ratio sensor can cause big problems with expensive catalytic convertors, but a loose gas cap…not as critical. The problem lies in not knowing if it is critical, until it is checked. We always recommend at least getting a diagnostic test run so you can make the best plan to fix your car and get that pesky check engine light turned off!! Hope this helps a bit! Thanks for reading Ask the Barnes Brothers! B&B Automotive Services where we treat you like family!