engine light came on-2000 honda accord?

EGR insufficient flow is caused by egr passages clogged due to lack of oil change, you have to clean egr passages. The code that you have is P0401 (insuffient egr flow) not P0420 (catalytic efficiency)

Bad exhaust flow is more than likely exhaust having trouble getting out of the engine...namely a converter going solid.You should have a serious performance downturn,and a top speed drop (like to 55 only).You should also notice a big drop in the exhaust sound at the tailpipe....like someone has shoved a banana up it.This isnt going away and needs to be attended to.Any good garage can do a back pressure test (they put a gauge in the oxygen sensor hole) and tell if you need a cat con.I would also question your mileage (160,000?),if its low,you may have a converter fouled due to a bad running/oil burning engine.

More likely a plugged catalytic converter. Until the problem is fixed, clearing the code is a waste of time.

The E.G.R. ports in the intake plug up. Honda has a T.S.B. about it. want you to replace the intake but you dont have to. take it apart and clean out the ports

Undo the gas cap and then reinstall and make sure to get at least 5 clicks. This should solve your problem

I agree, sounds like a converter to me.

Have your catalytic converter checked and consider possible replacement.

go to Autozone and ask them to pull the codes for you. They can tell you exactly whats wrong for FREE