Ok, got a P0400 out of the car. In fact, it seems to be throwing the code every 10 or less miles!

I understand this to mean that there is a "malfunction" in the EGR path. But not necessarily low flow (which would be P0401) or too much flow (which would be P0402).

In fact, there are nine codes for EGR trouble.

Two questions:

1) What is it really trying to say?
2) How does the system know if there is too little or too much EGR flow? How is it measured?

It seems like a simple system and I get the idea that replacement of the EGR valve and Solenoid assembly ($120 for the part at AutoHaus) would probably fix it. But I'd like to understand the system more before I proceed.

The car has 73,000 on it and I've been driving it more spiritedly the past 10,000 or so. I suspect I've blown some crap out into the exhaust manifold and some of it has passed into the EGR valve and fouled it.