Diagnosing o2 Sensor

Hey, So using a Haltech Sport 2000 on a RB25DET I noticed the idle was quite lean (car had been tuned previously)

So I tried adding abit of fuel to that area of the map and it richened up... for a second then went lean again... Repeat, same thing... Hmm So I bought up the o2 correction % gauge and the o2 was removing 10% fuel sending the car lean and fighting me....

I turned o2 correction off and adjusted to get the afr's to 14.7 with my wideband and watched the narrowband o2 voltage gauge and it sat at 0.983 Volts.

It was a new sensor about 6 months ago, 4 Wire... What could be the cause of it being stuck at .9v?

There are a variety of reasons for the sensor to die. Exposure to moisture, leaded fuel or even running excessively rich will damage the sensor. Personally I'm not a huge fan of doing just about anything with a narrow band sensor though. Id probably suggest your customer consider something like the Haltech CAN-based wideband unit for accurate control.