Toyota Repair: 98 Camry CEL, vacuum hoses, modulator valve

Question
Hi Ted, a friend of mine has a 1998 Camry, 4 cyl 2.2L, and the check engine light has given me a P0401 code (Insufficient EGR Flow).  Without taking anything apart (yet), the EGR seems fine on inspection, it moves with slight pressure and returns to place, vacuum hoses on top seem fine.  I've read other causes for this code may be EGR position sensor (I beleive on top of the EGR valve???) or a DPFE sensor (big question mark on this one).  My question would be what approach to take to correct this code, and what/where is the DPFE sensor.  Your help would be appreciated, thanks!!

Answer
The 4 cylinder engine doesn't have a position sensor, the most common cause for this code is the egr modulator valve located directly on the egr valve, in most cases replacing the modulator and the egr vacuum switching valve takes care of the problem.