I recently asked a similar question myself. Turns out the O2 sensor (if you have the OEM three wire one) has a built in heater of its own, so it is independent of the coolant temperature. If the built in heater goes bad, it takes longer for the O2 sensor to get hot enough to work properly.
If you have an after market one wire O2 sensor it takes longer for it to get hot and it robs you of fuel mileage. In fact I am not even sure the one wire O2 sensors will even work with a Renix. Probably not.
You ask what could be the problem, but are you really having a problem? Don't trust the temperature gauge on the dash. They are not precise, or terribly accurate. Where are you reading the 170, and are you using a seperate temperature measurement device to verify the temperature?