Schematic / Diagram for ECM in 2000 Bonneville SSEi

Hello.
I am having problems with a reprogrammed ECM in that the #3 cylinder appears to go lean at full throttle.
Coil, plug and injector have been replace leaving only the ECM.
Since the ECM has a modified prom, I need to repair it if possible.
Is there a source for diagrams, parts etc?
Is there a repair facility other than GM?
Thanks
Ben
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