I was having a difficult, more like impossible, time solving a constant egr code (P0401) so I went back through the various posts and found that duke17 had a similar problem with his 98. He solved it by replacing his 1500 ecm with a 2500 ecm, blocking off the egr valve and leaving all the wiring intact.
I 've done the same with my 97, no more egr code, but the "wait to start" (WtS) light does not come on when I'm starting the truck but comes on and stays on when driving. It goes out when there is no load on the engine, ie backed off on the fuel and coasting. Cruising down the freeway the light is on. Under hard accelation it goes out briefly but then comes back on.
When starting the voltmeter on the dash behaves like the glowplugs are active(voltage drops once or twice once started) but there is no drop in voltage noticed when driving.
The ecm from the 2500 is from a truck with all the same features just a 3/4 ton.
I'm just about ready to put a bullet through the block. I've spent big coin trying to solve this problem and none of pro's seem able to help.
ANY AND ALL THOUGHTS APPRECIATED.
alex
PS Noticeable improvement in power, mileage also seems slightly better

Ran that way myself for a long time until a viable OBDII reflash came out, did you do theft deterrent relearn, and what year was donor 2500 PCM, never tried a non theft deterrent PCM in a deterrent truck may be some PCM comms problems there causing lite, see if you can get someone to read the VIN stored in the donor PCM with a T2 or Snap On scan tool,

No theft deterrent in the truck and the ecm came from the same model year. My truck was built in late 96 but a 97 model year.
Tell me more about the reflash. Valley Fuel Injection/BD did not seem to know if that was possible. They provide many aftermarket diesel products but have been of no help with egr code problem or knowing if the existing 1500 computer could be reporgrammed to the 2500 series specs.
thanks

Reflash is a reprogram of the PCM for more boost & fuel delivery, GMs stock programming is very lame, go to FAQs and read up on how to make more power with the 6.5, mostly it's how deep into your pocket do you want to go, looking from your sig, you need gauges before doing much more for power with your truck other than the F PCM swap which nets a little more power, 63mm peak fuel vs 56mm in a 1500s program.

If you are not ready for a hot aftermarket chip, getting the L65 program is one heck of an improvement.