I'm getting this code with no check engine light but the truck does not idle as well as it used to. That is the only symptom. P602 is a programming issue with the ECM. Any ideas?

Smarty... It's causing the error code... If you reset to stock and clear the codes you'll be fine... But as soon as you run a Smarty program the error will show up... Of course because you modifying the stock fueling tables in the ECM and the checksum doesn't match any longer...

So do you suggest this may not be causing the odd idle? Im considering putting in newer injectors with the hopes it will take care of the minor idle issue. And I did try that but nothing changed. I had problems with my smarty at one time which is probably why. I got stuck in a tune.

The 602 is known as "the smarty code" . Do you have a edge box ? Did you check the apps?