This is my DIY for clearing the CEL code P0401 (EGR Insufficient Flow) on a 98 Honda Prelude.
In cars with OBD2 if the EGR valve is restricted by carbon deposits it will detect that and throw a CEL. To get rid of that CEL you must clean out the carbon to allow flow.
After combustion occurs within the engine it creates exhaust. Gas from the exhaust straight from the engine is very bad for the air. The EGR valve recaptures that exhaust and runs it through the combustion process again, which lowers the temperature of the fumes (exhaust gas) and lowers the amount of pollution exiting into the atmosphere.
If you don't fix it you won't pass SMOG in California. You may experience rough idling and poor acceleration.
Get your "EGR port plugs" from Honda. They will have it.
Removing the injectors are important to allow clearance to the port plugs.
When you pull out the injectors you might as well clean them since they are out.
Slide hammers make pulling the port plugs out easily.
Drill with a bit the size of your tap. Drilling halfway through the port plug to should be enough distance to fully grab the port with the slide hammer.
If after you clean your ports and valve the CEL has not been cleared, you EGR valve maybe faulty and needs to be replaced.