There are no top reasons. A simple scan of the ECU with an OBD2 scan tool will retrieve the code and tell you what part is being effected. Most auto parts stores will do this for free.
Check your fluid levels.
There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.
I am not sure of the other 4 reasons But in my experience the top reason your check engine light comes on is because your gas cap isn't tightened all the way.
check power to fans,if power check fan sensor!!
Fix the problem causing them to be on.
There is an engine misfire, probably spark plugs issue.
There should be a dipstick like the one for oil. Check with engine running.
Get the computer scanned to see what the problem is.
The light could come on for a number of reasons. The vehivle would have to be hooked up to a scanner to determine the engine code or codes. After you determine the codes you can isolate and fix the probelem. If the check engine light is on there will be a computer code that will appear on the scanner.
Disconnect the negative battery terminal for at least 30 seconds.
(torque converter clutch circuit performance or stuck on)
Check your coil pack or the spark plugs.