Engine running due to stall converter???

Myself I would go with the infrared gun. Seeing as how this is one of the tools my wife bought for me which means it's not a Harbor Freight piece of crap LOL like I would buy. You have to make sure the sending unit is of the same manufacturer as the meter inside the car? I just recently had the same thing happen when I took out my Stroker motor and put on a intake manifold with a sending unit already in it on a different little small block. The temperatures were skying through the roof instantly but my infrared again was showing the same temperature as it always did before with the Stroker motor. I pulled the sending unit out of the Stroker motor and put it in the new motor and bingo! Everything was showing the same on the gun as it was on the meter in the car again.
Infrared guns are very easy to check LOL pointed at yourself. Most people know what temperature they should be.