The temperature gauge sensor is combined with a switch in one assy. Both this switch and sensor operate the dash lamp and temp gauge respectively.
gauge/lamp sensor dk grn 84-88 all 25036628 tan L4-LH front of head 85-88 all 25036809 dk grn/wht V6-LH rear head next to edge of exhaust manifold
I have found that the connector shell on the V6 gauge/lamp sensor is melted or gone due to the proximity to the exhaust manifold. The wires can be simply slipped on the terminals without the shell. How do you tell which wire and/or terminal is which? Easy. Install either wire on either terminal. Run the car. If the gauge doesn't come off cold, reverse the wires. To check the gauge and lamp, short either wire to ground with the ignition on. The gauge should go full scale, and the lamp will illuminate, respectively.
From: Tin Man
The Fiero uses two different temp sensors, one for the gauge and one for the computer. The gauge is not vital to the vehicle's operation. If it is not exact, you'll just get a false reading (and maybe get alarmed). Checking this is easy (just to be safe that your sensor is to blame and not the gauge). Carefully remove the engine temp sensor's connector and put a 55 Ohm resistor across the two contacts (22 in series with 33). Read the gauge. It should read 260. Now connect 1400 Ohms (e.g. 1000 in series with 390) across the contacts. The gauge should read 100. If it doesn't, the gauge may be defective or the wiring has a high resistance.
From: Oliver Scholz
My temp gauge started to behave out of character, mainly reading a lower temp than it should have. I replaced the temperature gauge sending unit, which is located by the trunk/driver's corner of the engine, near the exhaust manifold, coming out sideways. It is extremely easy to change using a deep socket (size forgotten). Make sure you remove the old one when the engine IS COLD, and don't open any cooling caps. Have the new one handy, so when you unscrew the old one, quickly screw in the new one. The new unit has the thread sealer already on it. Almost no coolant drips out. The job takes about 3 minutes, and $18.00 for the sensor.
From: Ron Dittmer