Coolant

I would suggest installing a sacrificial anode if you have not done so already. A little extra insurance never hurts.

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Originally Posted by firebird77clone

Seems to me, before you could verify sending unit (by measuring resistance) you would first need to verify temperature. So the afore mentioned heat gun is a good start.

Once you know the actual operating temperature, you could then test resistance of the sending unit to know which is bad, the sender or the guage.

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I put one in boiling water with ohm meter leads clamped to it. We know that temperature.

Teflon tape or sealer on the threads------------

If everything checks out you might check because steam also doesn't conduct heat very well, if you have the sending unit in a npt reducing bushing you could have it in a air loc or air bubble measuring steam or air temp and not water temp, thats what was going on with mine, I loosened the sending unit until water came out and my problem was cured.

I'm with Jerry on this, usually air/steam pockets make it read on the high side. The thread sealant increases the resistance, the sending unit is a heat sensitive resistor the more resistance the cooler the gauge reads. Of course there's always the possibility that the gauge is out of calibration or it has a mismatched sending unit.
And as mentioned previously, real easy to check the actual temperature with a laser, most shops will read it with their laser for free (or very low fee).