There are a number of things that could make the light come on including you coolant temperature sensor, very easy part to replace, check your gauges and make sure that all of yours are working and that they are reading correctly.
How do you reset check engine light for the 2009 Chevy trail blazer?
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.
Check for vacuum leaks.
Check the motor mounts.
Random cylinder misfire
Check the engine thermostat. Its probably stuck open.
Bulb may be blown or has been removed.
The check engine light is triggered by the Engine Control Module (ECM). The ECM receives information from many sensors and determines when to illuminate the check engine light.
Check for vacuum leak
The transmission fluid on a Chevy Blazer can be checked by idling the engine and allowing it to reach operating temperature. The dip stick can then be removed and checked based on the markings provided.
check for vacuum leaks that wull usually cause this problem
You fill in the same place you check it at the top left above your engine