Have you looked at scan data for the o2 sensor? Does it toggle rich/lean? If not...have you tried forcing it rich/lean to see if it reacts? Almost sounds like an open in the 02 signal circuit which would put the signal voltage at roughly 450mv. From Alldata:

SYMPTOM
P0134-1/1 O2 SENSOR STAYS AT CENTER

WHEN MONITORED
Engine running for greater than 121 second. Coolant Temperature greater than 66 °C (150.8 °F). Engine in closed loop fuel control mode.

SET CONDITION
O2 signal voltage is between 0.35 volt and 0.58 volt for a total of 30 seconds and than O2 signal volt is 1.5 volts for 60 seconds. One trip fault.