Trouble Interfacing Ultrasonic Module USSM1.0

I am trying to use Ultrasonic Sensor Module B59110w2111w032. I have connected BSS138 (N-FET) for level shifting from 12V to my 3.3V system and viceversa.The pull up on Sensor side is 6.8K and connected to Drain , Gate is connected to 3.3V and Source is pulled with 10K to 3.3V on my board side. I found that after issuing first 100uS trigger pulse the IO line is not pulled to zero by the sensor. It remains at about 2V and so I am unable to detect the low edge on uC side.. Pl help with sugestions. Is anything missing from my side.