1997 Ford Thunderbird

On the first startup of the day when my car is started, the Exhaust is smoky until the car reaches normal operating temperature. Also my check engine stays lit. I have had two different mechanics that can’t find out the problem. All sensors seem to check out but the computer does give an error code. The plugs are clean and there seams to be no bad residue
when I wipe the inside of the Exhaust with my finger there is hardly any residue


Thank you,
Dean