The check engine light comes on because there is a problem with one of your car’s systems. It could be something very minor or something potentially damaging. The problem is that a car’s computer is very sophisticated, and it is able to compensate for a problem in one system by changing the parameters of another system. This results in a ride that feels so similar to how the car normally runs that the owner might dismiss the check engine light and leave it undiagnosed for a few weeks. This can cause stress in the compensating systems and could make the vehicle very vulnerable to damage.

Of course, this is not always the case. Something as simple as not tightening the gas cap enough can cause the check engine light to come on. Since there is no simple way to diagnose the problem without the appropriate tools and knowledge it is best to have it checked, even if it is between your regular maintenance interval.

Why Not Use a Free Service?

It is best to pay to have this service done for you by a trustworthy technician and not have it done for free at an auto parts store. Let’s say, for example, the code comes up that it is the Oxygen Sensor, an auto parts store might just sell you a new oxygen sensor, meaning that their “free” diagnostic test actually cost you an expensive part. When you pay a moderate fee to have the diagnostic test done by a trained technician, you get knowledgable analysis of all of the possible causes of that error message. They might be able to see that the oxygen sensor warning has been caused by a simple vacuum leak.

Engine diagnostic test at a shop: $25-45

VS

“Free” diagnostic test + new Oxygen Sensor at a parts store: $150-300.

In the end, just find someone you trust who does not have an agenda to oversell you. If a service is “free” it might mean they are over charging you in another area to make up the expense!