Weird idle behaviour on startup after replacing o2 sensor
 

I just finished replacing the clutch on my 924s and while I had the exhaust down I decided to put in a new oxygen sensor since it had never been done before. Starting it with the new sensor now I notice that the idle will surge from about 900 to 1200 rpms for the first minute or so on cold startup and then return to stable idle once it has ran for a bit. Starting warm it does not have this problem. When I unplug the o2 sensor on cold startup I do not have the same issue and the revs remain consistent throughout the warmup period. It's a brand new Bosch o2 sensor the same as what I pulled out. Heater circuit on new sensor has good continuity and the voltage does increase with temp. Anyone wanna take a guess as to what's going on here?

That’s odd since I think that the O2 sensor output is ignored for the first minute since the sensor will not be up to temperature. Maybe a fault with the actual sensor not the heater.

Exactly what voltage are you measuring and how are you measuring it?