That is interesting as it is unlike what we have in the US, which I am sure has different emission standards than Costa Rica. One-wire oxygen sensors are the older style sensors before manufacturers started installing heaters in them to warm the sensor up quicker, allowing the ECM to enter closed loop quicker. Oxygen sensors create their own voltage by the heating of the precious metal(s) used inside them. The voltage produced will usually range from 0 volts to as much as 1.1 volt, cycling up and down according to the amount of heat in the exhaust. The three wire sensor is an indication that it is an oxygen sensor with an internal heater. Back before heated sensors, the connector would typically be a 1-pin connector, which is all you need since there is only one wire. FYI, Nissan stopped selling the Hardbody, unfortunately, in the US after the 1997 model year. We then went to the Frontier, which is known as the Navara in many other countries. My thinking is that your Hardbody still uses an OBD I system (the US required OBD II compliancy in 1996 for US sold vehicles to be operated in the US). Perhaps they went to the 3-pin plug design to cut down on the number of oxygen sensors they make or take advantage of the later oxygen sensors performance due to the precious metals used in it compared to the earlier sensors, but just didn't need the heater because it's an OBD I system? I can't really answer that. As long as the center wire is the signal wire, which I'm pretty sure it is, this setup should work and is likely correct. You can always check with the dealers in your country, who are more familiar with your particular vehicle. Whether the sensor is working properly or not, and possibly leading to it running rich, I cannot say, obviously. It's certainly a possibility. But I would also make sure that fuel pressure in in spec and not excessive (which would likely indicate a bad fuel pressure regulator) and the spark plugs are in good shape and properly adjusted, as well as confirming proper ignition timing, compression, etc. If it is an OBD I system, it will likely have a way to access trouble codes via a mode screw on the side of the ECM (if it's a 5-mode ECM) or a toggle switch (for a 2-mode ECM) and a pair of LED's, one red and one green.