Depending on the reason the check engine light was set in the first place, I would contact the repair facility that performed these repairs for you. Make sure you know the reason the first code is set and find out why it was not repaired correctly the first time.
First, I'd have to suggest finding out what type of code you retrieved when the computer was hooked up to your truck. This code will indicate what system the problem is related to. Depending on that code, you will need to run specific tests to find out what is triggering the check engine light.
〉 Answered on Sep 2nd, 2008 by Suzanne Grego, Technician at City of Philadelphia Fleet Management