What could be the reasons for the check engine light to come on and stay on?

You should have the vehicle scanned to determine what DTC's (Data Trouble Code) are present in the OBD-II system, and most auto part stores will scan it for free, (be sure to write down any DTC's correctly) and there just are too many possibilities to list because there are about 2,000 common reasons for the MIL (Malfunction Indicator Light) or "check engine light" to come on, and any one of those DTC's set in the OBD-II system will cause the MIL to stay on.

The check engine light is triggered by the computer receiving fault codes from various sensors/components. The light will not turn off unless the ignition cycles(starting and stopping the engine) around 40 times without any fault codes being detected by the computer. You can have the computer scanned at most auto parts stores. Auto zone, Napa, Checkers, Schucks, and Murray's will all scan your system for free to retrieve the codes present that triggered the engine light. You can come back here and list the code/s and we will try to help to correct the issue. Without scanning the system there is no way of knowing what caused it. You can also disconnect the battery to clear these codes but you wont know what caused them. I would recommend getting the system scanned, its free and it takes around 5 minutes.

If your engine is running different then before the check light came on. have your computer system checked for error codes. start there. or your owners manual. or a service manual at your local auto store.