How do i fix service engine soon on a 1999 Nissan Altima with a diagnostic error code p1400?

that's egrc solenoid valve, you can go and get it check at any parts store or buy a obdII code reader and erase it if the problem persist replace the egrc solenoid, and have the code erased again it should fix the problem, althougth i had a problem like that and it wasn't the egrc, was the egr valve since goes connected to the egrc can give you that problem chek if you have a code for egr, if it shows the code i will suggest to replace the egr it's cheaper