I am replacing the exhaust manifold. Old manifold has a some sort of sensor screwed into it, but the new one does not have a port for the sensor. The sensor you are referring to on the exhaust manifold is the oxygen sensor. It is an extremely important part of the computer management system for your engine.
Oxygen sensors read the fuel mixture in the exhaust and send the information to the computer.
The computer then, adjusts the fuel mixture. By not replacing faulty oxygen sensors, your can expect that your check engine light will eventually come on accompanied by erratic idle and poor running. Your car probably will not quit running and there should not be any damage done to it by driving it with faulty oxygen sensors. MOT failure on emissions - running at 10.30. Garage suggested several possible problems starting with Oxygen sensor - cost of repair 100.