i just bought a all original 10,000 mile 85 trans am with a l69 5 speed combo and have a little problem .. first thing is the car runs great but the check engine light is on and i get a code 23 .. so i start looking under the hood and noticed they had a sensor unplugged .. i plugged it back in and the check engine light goes off but the car wont run .. hear is the connector they had unplugged

 

That's the mixture control solenoid.Could be its not adjusted to spec anymore or it may just be faulty.It should make a rapid clicking sound when functioning properly, it should also do the same for about 10-15 sec when the key is on, w/ eng off.

Adjusting it will require removing the little cap (visible in the pic) at center, by the a/c gasket.Also the other cap like it directly behind & slightly higher.Those are your lean/rich mixture screws under the caps.Might have to adjust the idle air bleed valve too, under that flat metal piece w/ rivets on each side.

Replacing the solenoid will require removing the air horn & even adjusting them requires a special tool/s.I made mine from copper tubes though.The procedure is outlined in the service/chilton manuals.

 

Without the MCS connected (that little blue thing), the carb will run in full rich mode. If it runs fine without being connected and you have no smog regulations to worry about, then just leave it as is. However, from your photo, it appears that the carb hasn't been given any attention in quite some time. The MCS may be gummed up or just plain shot! It is probably a good time to rebuild it yourself or have it done. Depending on your level of experience, you can make that choice. The MCS is basically a stepper motor which raises or lowers the carb needles at part throttle operation. It doesn't even come into play at full throttle. When it is working, the carb will give the best air fuel mixture possible.

A rebuild will most likely have it running like new!