My 1998 F150 4.6 had the 401 low EGR flow code. Yes, the EGR ports inside the throttle body were badly clogged. Took it off, cleaned them out.
I see tons of questions like "what causes this code?" "How to fix it?", etc.
I want to know how to keep it from happening again? What causes the build up to begin with and what to do to prevent it?
Thanks

 

There used to be an add-on screen (@ throttle body) to prevent ingestion of large particles of carbon into the EGR system.

Maybe someone with an MPC can find the PN. Otherwise just clean it on a regular basis.

 

Accumulation in a clean system cannot be eliminated over the long term.
The exhaust is forced by exhaust pressure and pulled by intake vacuum through these ports.
Exhaust is already contaminated with moisture, cold rich running carbon, unburnt fuel, oil and acids from combustion.
Naturally this contamination settles on cooler surfaces from cold starts and normal running conditions.
For some of these reasons, during freezing temperatures, the flow test done by the computer to test the system for blockage is always done two times each drive cycle to be sure moisture has not frozen the system shut causing a fail code.
The second test is done after the engine bay has heated such that the freeze area has cleared.
This also can plug the DPFE sensor.
Short drive cycles cause most of the issue.
I have 270,000 plus and have not had a blockage yet.
Part of this effect can be seen in winter temperatures by seeing the tan colored goo coating accumulation on the oil fill cap for much the same reasons.
A 20 year old engine certainly can have an accumulation from the above conditions along with oil passing by the piston rings ending up in the exhaust stream as high unburnt carbon content in the black smoke phase.
Its also what kills Cat converters along with additives in the gas.
Only thing that saves the Cats and Ox sensors is the high temperatures they run at.
Good luck.