Check Engine Light reading DTC PO125 or P1135
 

Some good understanding followed by a Test..lol
http://www.autoshop101.com/forms/h37.pdf

The P1135 is a low current malfunction on the AFR/O2 heater circuit of bank 1 sensor 1. Ideally you need to test the voltage at the computer Pin HAF/O2 it should read 12 volts. With in a minute or two the voltage should drop to 0 to 2 volts when the computer turns the ground circuit on to energize the circuit and heat up the sensor. Then it will toggle between 5 to 10 volts depending on the heater circuit demand. If the voltage is not 12 volts then you need to check the voltage at the sensor. If the computer never drops the voltage then the driver is bad in the computer. 90% of the time the AFR/O2 sensor is bad but in some rare cases I have seen them take out the computer also. Bank 1 Sensor 1 is the sensor in the manifold by the firewall. Make sure you check the 25 amp A/F heater fuse.

All of this being said the easiest way to get the sensor out is from under the vehicle. Disconnect the electrical connector that goes up by the intake manifold. Using a 22 mm wrench or a 7/8 inch wrench slide the box end over the electrical connector so the wrench is pointed/angled slightly forward. At this point you will be slightly parallel with power steering rack and it is very tight but you can break the sensor goes and work it out. There are some specialty tools that you can buy that will fit on the sensor but I use a box end wrench all of the time and it works.

If the sensor starts to come loose then gets tight do not force it out put some brake fluid on the threads at the manifold and leave for 5 min. then work it back and forth and it will come out. FYI