Toyota Repair: 99 Avalon MIL with Codes P0125 and Pending P1133, o2 sensors, coolant level

Question
Hello Ted,

I hope all is well.  I've got an '99 with 19K miles (Mothers car not driven much) with a MIL on.   The MIL came on right after a fill-up  (maybe coincidence) with the following codes P0125 Insufficient Temp for closed Loop and Pending Code P1133 A/F sensor Circuit response Bank 1/Sensor 1 . Gas cap is tight, coolant level is OK and engine comes up to operation temp normally. I would like to get some of your expertise on how to diagnose and test the sensors. Also, where the sensors are located . The A/F (O2) sensor indicates upstream but is it front or rear bank?. On other cars I have measured the voltage of the  O2 sensors with a DVM between 0.2-0.8V at 2000 rpm.  I then pull a vacuum line to see how quickly the sensor responds. Thanks.

Answer
This is a common trouble code on these models, the solution is to replace the a/f sensor, B1S1 is located in the exhaust manifold in the cylinder bank near the firewall, the sensors also have a heater circuit, a total of four wires so when checking voltage be sure to check the correct wires, the reference voltage is at 0.5 volts fluctuating to +1v to -0.5v, replacing the sensor takes care of both of the codes.