I show a 2004 Jeep Liberty Renegade, but this will also work for a 2002 - 2007 Jeep Liberty.

Steps:
1) Put your key in the ignition

2) Turn your key to the accessory position x3 times (all within 5 seconds)

3) Code will appear where your odometer is

If, for some reason, this doesn't work, you can stop into any Auto-Parts Store and let them know that you have a Check-Engine Light. They should have an OBD2 scanner that they will use and check the codes for you.