What might cause my 2001 Ford Explorer check engine light to come on?

There are a whole bevy of reasons You might have the check engine light (CEL) illuminate on Your 2001 Ford Explorer. The best thing to do here is find out what diagnostic trouble code is being thrown concurrently with it: You can take Your Explorer to a local AutoZone to have them read the code for You, or You can buy a scanner (usually around $50), or a bluetooth dongle (< $15, typically) that You pair with Your phone to read the code. When You've got that, You'll know at least what the engine thinks is wrong, and be able to troubleshoot from there. Typically, though, this can be anything from an oxygen sensor failing, to an issue with the mass airflow sensor, issues with the fuel injection system, or more serious issues like a mIS Firing engine.