oxygen sensor related to coolant DTC?

Any thoughts?
History.. I had a toasted cat. Was told my 02 sensors cat needed to be replaced by mechanic. I replaced the cat through the dealer under warranty but never changed the o2 sensors. engine DTC/CEL were cleared. all good for few months. then CEL on again. figured it was for the o2 sensors acting up so i replaced the front (precat) o2. (tip was real white) (150000km so figured needed it anyway). but CEL stayed on.
where i am now...
so i just ordered a rear o2 but i also bought a ELM scantool for my laptop and actually read the code which says only p2181 (coolant problem).
so i thought hmm either a) maybe the front o2 code corrected itself when i replaced it and the rear is fine, so now theres just this 2181 code. or b) the o2's were fine all along and i just have a 2181 code now. or c) the scan tool is missing some of the codes (unlikely).
data from the scantool:
short term fuel trim (bank 1): +1.6%
long term fuel trim (bank 1 ): +0.8%

intake air temp: 33 deg. C
Coolant temp: 68 deg. C

sensor 1 bank 1 : N/A
sensor 2 bank 2 : .475 V
Question:
this is the only data i have from the basic scan tool. the voltage on the rear o2 appears fine.
why is the front o2 sensor reading N/A ? if it was not connected properly, it should throw a code shouldnt it?
im not having any temperature problems that are noticeable. the car was not quite warmed up when i took readings. (temp gauge was about 1/4 way up. sits at dead centre when warm)
i dont know anything about fuel trim data just included it for FYI.
Aside from a possible thermoswitch/thermostat issue (p2181), i'm just wondering if i should be concerned about the N/A o2 sensor data? any logical explanation for that?
sorry for the long post, just trying to give adequate info.
cheers,

2.0L golf 2003



Modified by firehat at 7:48 AM 3-20-2010