P1640 can be set by more than a bad PCM.
it'* a DTC for the quad driver, and any kind of circuit failure in any of the circuits that are controlled by that specific quad driver can set that DTC. the boost control solenoid(if it were present), the EVAP cannister solenoid and the TCC Apply solenoid(the one connected to pin E12) are the ones connected to this quad driver.
any type of manual override, shorted circuits, open circuits, weak circuits or bad solenoids would be what i would look for.
1995 Monte Carlo LS
3100, 4T60E, OBD1 Conversion...for now, future plans include a 3900, T04E-46 (Knock-Off) turbo, and a F40.