How to fix if check engine light is on

I'm not sure where in the world you are, so I can't comment on regulations for you, but here in the US, all cars made since 1996 are required by law to support the On-Board Diagnostics protocol, more commonly referred to as OBD-II. This specifies a standard method for communicating with the car's onboard computer(s) and what information can be returned.

The OBD-II standard specifies that certain problems detected by the computer system must turn on the Malfunction Indicator Light (MIL), aka the "check engine" light. These are the more serious issues that can cause emissions problems or damage to the car's mechanical components.

So the fact that the light is coming on means that the car's computer is trying to tell you there's a problem. If it's flashing, that's almost always a dire warning that you should not drive the car! There is no way of telling what the issue is simply from the fact that the light is on; the car's computer has to be checked with an OBD-II scanner; usually a handheld device that plugs into the OBD-II port located somewhere in the driver's compartment.

The scanner will read out the Diagnostic Trouble Code(s) and report them back to you so you can tell what problem caused the MIL to turn on. These DTCs generally start with the letter "P" followed by four numeric digits. Most are common across all cars, but many can also be models-specific, so you have to look up the code together with knowledge of the year and model of the vehicle.

Scanners come in a wide variety of options; there are the professional ones that can read very specific and proprietary information from every car model available and can cost several thousand dollars, all the way down to a Bluetooth connector that interfaces with a free app on your phone and can be purchased for under $5 USD.

So when that light comes on, the first thing you need to do is find out why it's on. You can either buy yourself an inexpensive (or expensive, I don't know your situation) scanner, or find someone with a scanner that can check your car. Here in the US, most auto parts stores will plug their scanner into your car for free and give you a readout of the DTCs they find, but again I don't know your situation. You mentioned your repair shop; they will most definitely be able to scan the car and they should tell you what codes they found.