Check Engine Code .....help please

[Mike Klein]

My wifes '01 X5 (E53) 3.0 6cyl, auto ( 1 owner, good services, 113k miles) is throwing a CEL code #0128 ( Low temp sensor reading) and #0141 ( O2 Sensor Fault, second sensor) both codes at once, even after reset.

I noticed on a long freeway cruise at about 65 degree air temp. that the temp gage was reading just below the 1st mark (less than 1/3 up) whereas at idel or around town, it tends to sit right in the middle, straight up. The pusher fan cycles on/off as normal. This would imply that the thermostat is stuck open. But I replaced that T'stat for general goodness about a year ago.

So here are the questions:
1) since I don't drive it much, I don't know what temp reading I should see on the gage. Help?? Can't tell if it's a faulty sensor or a bad T'stat.
2) could the O2 sensor code be related to the low temp?
3) any cheap mail order sources for parts??

Any other help would be appreciated. THANKS.

Mike