There is a cyl head temp sensor behind the timing cover on the left side cyl head. Many times the connector has corrosion or the wire breaks off. Some nissan dealers still have that pigtail wire with the two plugs you'll need along with a new temp sensor. That would be the most likely cause. The black square connector is on the front of the air intake passengers side and the Ohms resistance is about 800,ohm at 70 degrees open circuit would indicate about -40 degrees making the engine mixture very rich.
