The DPFE. They commonly get a small amount of water in them to corrode and fail, water vapor is present in all engine exhaust. The sensor actually only has a layer of RTV maybe .010" thick to protect the sensor strain chip, eventually the water gets through that.
I had trouble with a newer design on my '02, the '00 never has had trouble at all (firewall mounted with longer hoses). The '02 failed two sensors, the very short vacuum lines (2 inch long) had something to do with it I thought. Too easy for water to get into it, too short a path. The bad DPFEs both had water droplets in them when disassembled. I used 6 feet of vacuum line cut in two for 3 foot pieces and ran the extra line down back of motor for moisture trap, also remounted the DPFE up higher to the intake tube, now water has to go up a long distance to get to the sensor. The sensor used to freeze water in it too when temps went below freezing, the motor would cough and spit since EGR was on at cold idle, a big no-no. Since the mod several years ago no trouble at all, problem cured.