GMC Repair: Check engine symbol, engine gasket, senser

Question
What does the check engine symbol look like? What does it indicate should be checked?

2000 GMC 2500 Sierra 6L V8 4x4 model: Long box.

Had to replace engine gasket when it was still under warranty. Gas filter replaced 2x. Recently have had extreme difficulty with sporadic weakening of compression. Often unable to go up a steep hill at a faster rate than 15 to 20 km. But sometimes it's okay. I live up a steep hill.

Rough starting when left overnight sometimes. Then warms up and goes smoothly.  

Answer
hello hun
the sybol is a yellow light on the dash . it also means it is time for a diagnostic test . which means you have a bad senser . egr valve and 02 senser this is just a few of some . good luck to you hun and if u need more help just let me know . And you have a pretty name pete