I must be missing something. I have an 2003 Silverado 2500HD w/ LQ4 4L80. I am a dealer for SCT and wanted to tune it with that software. Problem is their device can tune all years before and after but not 2003 ECU's.
So I bought an ECU from a junk yard that I verified to be out of a 2004 Silverado with the same engine+trans. But swapped out the ECU's and now the truck turns over but never fires. Did I assume too much here? Maybe the ECU I got is fried?

 

i believe that once you take the one out of your truck and replace with a diff truck it wont work. im not sure if theres a re learn for this or not

 

Might need to do a crank relearn, but it's not very common. Is your security light blinking when you're trying to crank it up? Any codes or anything?

What if you put the old PCM in, will it fire up then?

 

i bet it's VATS - try a security relearn. like others said - is it flashing a security light on the dash?

if not, check that your coil packs are securely plugged in. hopefully, it ain't like my engine swap when something was whacked in the tune because the computer wasn't telling the coils to fire - i had to sent it back & i don't know what he did, but when i got it back, it fired right up.

 

It seems to be the VATS. I'm getting a P1631 code (VATS code). And the security light comes on.

I just did one of the relearn procedures and it didn't relearn.
Key on 11 min
Key off 30 sec
... on 11min
... off 30sec
... on 11min
... off 30sec
then try to start.
Didn't work.

But on the Vette forums one guy said to do that procedure but try to start the car every time your turn the key back on. Trying that right now. 22 more minutes.

 

http://www.nelsonperformance.com/passkey.htm

 

Thanks guys. Works now.
For future reference- You HAVE TO try to start the truck every time you go 'key on' during the 30min. procedure.

 

Vat's
You need to do the relearn procedure