Investigations so far:
Sensor large thread and small thread checked for isolation. Tick.
Added earth wire from large threaded part of the sensor to an earth stud in the engine bay. Resistance between sensor thread and engine block 0 ohm. Tick
Connected sensor-gauge wire. Resistance between sensor small thread and large thread now 4-5 ohm. I have a circuit (but is this resistance too high/low?)
Sensor end plopped into cup of hot water abs ignition on. No movement on gauge needle.
Swapped light and gauge positives to rule out wiring loom problem - light still comes on.
Started engine to rule out a voltage issue. No movement on gauge needle.
Stumped now. I see four root causes - faulty sensor, faulty gauge, voltage drop, installation problem.
Any suggestions on how to clear these out?