2001 LS6 manual. It starts and 'runs' for a few seconds, then stalls. Consistent failure. This is an engine I just finished rebuilding and reinstalling in my car. No CEL light or DTCs using scan tool. However the Forscan software shows codes 1237 out of the PCM and 1641 out of the REM. One says fuel pump primary failure the other fuel pump secondary failure. The fuel pump is new. It MUST be pumping some fuel since the car does start. The crankshaft sensor output MUST be good since I get data on the tach. The relay, fuses and diode associated with the fuel system all test good. If I pull the MAF connector engine runs for a few extra seconds, then dies. Any ideas?