2.3v or 3.3v? ECU will send a voltage down to see if it's being pulled down for diag purposes. Newer stuff is definitely 3.3v, but I can't say I've needed to test that much on these.
All the ones I've changed have been intermitant failures but resistance on the sensor has always been over 1000Ω (Kia spec: 860Ω±10% @20°C).
I'm assuming the code you had was P0335 (not P0355)?

I've emailed you a little more info on P0335.

Tony

Note: The EGR valve can stick open, and will NOT log a fault code!