Sometimes you need a scanner to read the diagnostic fault codes. Most independent shops have at least one, but if there is none in your area, the dealer can read the codes. It should only take a few minutes.
There are well over 2,000 potential fault codes, but we can narrow the cause down by your dandy observation the check engine light came on right away. That eliminates anything that needs some drive time before the problem is detected. Most commonly this will be an electrical circuit defect. The tests for most electrical circuits run as soon as you turn on the ignition switch.
There are a lot of things the engine computer checks for that do not affect engine performance. Most notably is the vapor recovery system that includes the gas cap.
There are two important points to be aware of when ignoring the check engine light. The first is the computer constantly compares various sensors and operating conditions to each other to figure out when there's a problem. For example, it knows that when the engine has been off for at least six hours, the intake air temperature sensor and the coolant temperature sensor had better be reading the same temperature. When it detects a defect with the wiring to one of them, it knows that sensor cannot be relied upon for a reference for other tests, so many of those tests will be suspended. If a second defect develops, it may not get detected and no fault code will be set. That new problem could affect engine performance, but with no code, there is no way your mechanic will know where to start the diagnosis. He is going to have to diagnose and repair the first problem so the tests will resume, then, once the new defect is detected, which could take from a few minutes to a few days, you will have to return to have that one diagnosed and repaired.
The bigger problem is when the check engine light is always on, you will never know when a new problem occurs, especially if it does not present any symptoms. Many of those potential problems can be very minor, but if ignored, can turn into very expensive repairs. This often happens when something causes too much gas to go into the engine. The cause can be as simple as a defective sensor, but that excess gas burns inside the catalytic converter and can overheat it. If the catalyst melts, the exhaust is blocked, and the converter has to be replaced. That is usually a very expensive repair.
There is plenty of other equally minor defects, but we have to know the diagnostic fault code numbers to know where to start. You might find more information from this article:
Since I already mentioned the vapor recovery system, here is an article that explains more about it:
Thursday, August 9th, 2018 AT 9:49 PM