The un-conspiracy-pilled response:

The CEL (Check Engine Light) comes on when the engine computer detects an error in the data from one or more of the engine’s sensors. It can be something as simple as the engine is stalled, or you forgot to tighten your fuel cap, and it can be something as severe as engine knock/misfire (typically the light flashes at that time), catalytic converter meltdown, or some other major hardware failure. Use any scan tool to check the codes, then lookup the codes to see what they are. It’ll give you a good idea of what is wrong with the car. Sometimes its a corroded sensor wire, sometimes the sensor has gone and disintegrated itself, sometimes there is a hardware failure in the engine and the sensor data is abnormal because of it. Use the Torque app and a cheap obd2 reader compatible with your car, or a dedicated code reader / scan tool from the auto parts store. You can also reset the light with the tool, but if there is an actual problem it will show up again after a bit.