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