1997 Cadillac Deville

You are not going to replace the evap solenoid circuit. Which solenoid are we speaking of, the vent solenoid, or the purge solenoid?

my car failed emissions the code was 1645 which states evap solenoid ouput circuit so dont know how to fix the problem

P1645 is relevant to the EVAP purge solenoid. It could be PCM, purge solenoid, or circuitry. First, check the ECS fuse in the underhood fuse block, and the pink wire at the purge solenoid. It receives power from this fuse. The PCM controls the ground, without being able to cycle the output you would not be able to test the PCMs ability to control the solenoid. If there is power to the solenoid, the next step would be to check continuity of the dark green wire from the solenoid connector to PCM connector C2 pin 24. If there is continuity, I would start with a solenoid.