there are two temp sensors in the u13 (sr20), both located on the drivers side rear of the engine (near the oil filter). they're relatively obvious once you take a good look back there, one for the gauge and the other for the ecu.
the gauge one i'm pretty sure is the outer (towards drivers side) one, iirc it's a single wire, changes its resistance to earth through its body depending on temperature.
mine was fluctuating at certain revs, the connection gets a bit weak (on the sensor itself, not the plug, it's a silly setup when you look closely), my fix is i've pulled the cable tight and cable tied it off so it stays tight, no more issues.
you can see the sensor plugs in
http://www.clubpoint.net/bluebird/intake/Img_5370.jpg on the left, just above the oil filter