Looks like you have a 5th gen so that means you car is obd2. On an obd2 car its hard to run that manifold because it doesnt retain egr. What people will do alot of times is leave the egr plugged in to the harness to try and trick the ecu in the car that egr is there and working which is what someone probably attempted on your car. Even with obd2 tho the ecu still will usually have a check engine light because it is able to sense that the egr is not installed and working properly, even tho it is plugged in. If by chance it is workign properly, unplugging it will most deffinately cause a check engine light if theres not one already.
To be able to run the manifold without an egr and no cel's you would need to convert to an obd1 ecu such as a P28 and tune the car. The P28 ecu doesnt use egr so therfore it would not cause a check engine light.
hey man i got a question for a dealer? I just bought my prelud and it has a skunk2 manifold on it and there is a couple parts that arent connected .. First off my egr valve do i need it my cel is on but the egr valve with the tube and the wire harness is still connected . Wat should i do about it hanging by the tube right next to my cai.. Can i dissconnect it and if i did wat would i do with the tube that turns into the metal tube ?thanks ahead of time man