Code 334

Well, the TPS really has nothing to do with EGR operation, so rule that out.

To check the function of the valve itself, put a clean length of vacuum line on the EGR vacuum nipple and use your mouth to suck on the other end of the line. You should be able to hear and feel the diaphragm inside the valve "popping" open. If you can suck a steady stream of air through the hose (assuming it's a leak-free hose) or if the valve diaphragm doesn't move, you've got a bad valve.

If the valve tests good, the next culprit is the EGR valve position sensor. I know there's a voltage or resistance test you can do to verify it's functioning within range, but I don't know how off the top of my head. You could always just pull one at a junkyard for a couple bucks and put it on to see if it works.