I Replaced ECM in my 95 LeSabre because the check engine light was on (code 1640) it is still on now, but I heard I need to drive it around and cycle it so the computer can reset. Any advice on this??? Thanks.

Where did the PCM come from?

Junkyard

If the PROM in the one from the junkyard is not correct for your car, that may cause a problem. Unhook the battery cable. Remove the PROM cover off of your old computer, carefully lift out the PROM by pushing down on the two lift arms, and try it in the junkyard computer. Take a sharpie and place a dot of x on your PROM so you don't get them mixed up.

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.