Might be a simple as cleaning the one you have - remove the wiring connector, then pop it free from the tank. Be aware of a gasket, which may be pretty fragile given the age, etc. Be gentle when removing and handling so as to not fracture the stalk...the weak point is where it joins the top cap.
The two exposed tangs on the normally submerged probe should be clean/shiny. Scrape off any buildup. Might be a small hole in each one...clear those for good measure. Shine them up w/steel wool and reinstall.
If you want to experiment, grab an ohm meter and check for continuity from the cap connector pins to each related tang. Gently flex the stalk to check for intermittent breaks.
Also, note I've gotten false positives from time-to-time and when it wasn't just time to clean the tangs, it took a spare sensor to make things right again. They're not all the same length (and thus resistance), so if you source a replacement, be sure to match same for same.
Be sure the tank isn't full of sludge while you're in there. Confirm the tube to the radiator's filler neck is clear and the radiator cap is in good shape.
Still no joy, R & R the VMM (be careful, it's easy to break the plastic that holds the clips) and clean the connector and socket on the back. Still no joy, start inspecting the grounds in the engine bay, starting with the strap from the pass. side motor mount to the body, then the ganged grounds on the header panel near the headlights, then the battery and leads off of it.