Oxygen sensor location on 2002 Montero sport
 

We are getting a P0134 - Cylinder 1-4 heated oxygen (front) sensor no activity detected code. Where exactly is the oxygen sensor located that would set this code?

This would be on passenger side right after exhaust manifold and before small catalytic converter. You can probably see it from the inside of engine bay if you follow the exhaust pipes down. as far as removing it, your best bet might be from below the vehicle.
The "no activity" can be caused by faulty O2 sensor OR bad wiring. Trace the wire from sensor to the wire loom in the engine bay; unplug connector and with the engine running test to see if you have 12 volts on one pair of wires (heater circuit) and I believe about 1 volt on the signal circuit (both should be measure on the vehicle side of the plug). If you have voltage coming to the harness, it is possible that you have a bad sensor. If on the other hand you have no voltage - replacing sensor is not going to do anything. You need to check wires first.

Thank you Hunter.