Diagnostic Aids
WARNING: This page is about a different car, the 1998 Oldsmobile Cutlass and 1998 Chevrolet Malibu. However, it is still accessible from the selected car via links, so may be relevant.
With ignition switch in RUN position, check for voltage transitions on SPI data line. If only history DTC U2003 is set, problem may be intermittent. Try to recreate fault by wiggling wiring and connectors while performing test.