Code 31 help
 

Yep bought my Supra Sunday and here I am on Thursday trying to sort out a code 31.

The Facts:
1987 Supra turbo 5 speed
Motor has less than 1000 miles.
drove all day Sunday Monday and Tuesday
Gave code Wednesday after a 20 mile drive.
Checked the sensor ohms, checked out ok

What am I missing what are the chances of the sensor checking good but is really bad, could it be a error in the wiring to the ECU.

Please Help!