Jeep runs then dies
 

Replaced the heater core in my wife's 2000 Jeep Grand Cherokee Laredo. I made sure to disconnect the battery first before I started taking out the dash and steering column. Now, it runs for like 3-5 seconds and dies. The sentry key light does NOT flash...it comes on and stays on after the key is in the ignition for about 15 seconds or so. So, any diagnostic procedures I can do before I go ahead with replacing the computer? And, if I do go with buying one online, do I need to take it to the dealer to have it re-programmed?

Also, on possibly an unrelated note, the air selector knob (I don't know what it's called) doesn't switch between the different air options (defrost, feet, etc) as well. For example, when I put it on straight defrost, it comes out a little bit there, but still a bit from the upper face vents as well. Any ideas on where to start?

I would double check my work if i were you. Make sure everything got properly reconnected.

Yep, that's all it was. It runs, AND has heat!