Sure you replace right sender/sensor? Fiero V6 have 3...
If gauge "works" when starting the car...
Likely isn't anything in the S-column.
OE wiring makes the gauge to slam the "top" of unit.
Notice the coolant Light Does Not Turn On at same time? This is because of a "Defect" from GM that wired both wrong.
At the Sender plug, ground light wire to block/frame, light should turn on. Should not need to remove the plug to jump back of terminal.
Sender for gauge is Resistive to block. 1365Ω = 100°F/38°C 55Ω = 260°F/127°C
Test at sender needs to pull the plug a use above resistor or any value in between to see if gauge reads. Gauge wire - resistor - engine block
If nothing the pull C500 next to battery and test at C2 on body side.
Get resistors from old dead radios etc. Many newer things have surface mount parts and not useful here.
Careful remove w/ a lighter and small pliers.
Should easy find 1000Ω (Brown Black Red) and smaller google color codes.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave