I have a VDO CHT gauge and sensor. the sensor is the type that screws in under a spark plug.

There is a red and black wire coming off the sensor that connects to the gauge. That's it. How does the gauge get power? Instructions only show power to the bulb.

What am I missing? Also the gauge is at 200 degrees when not hooked up....

Can't find any explanation on how this would work.....also which plug is best?

Gary, I would check the accuracy of that combination before you install it. I've found VDO gage/sender combos off as much as 40F. However, with the VDO gage and Dakota sender it was dead on. Bench test it using oil in a can. Heat to 220 and check with a candy thermometer. Water will cool too quickly and is difficult to get over 200. You want to check your equipment at a temp close to what you will see normally.

Just guessing here, I don't have one, is the sensor a type of thermocouple? Hot joint and a cold joint? If so, the temperature differential creates a voltage, the gauge is analog (D'Arsonval movement, I assume), and the thermcouple generates enough current to drive the gauge.

This is the same principle that radioisotope power modules on deep-space probes use. Except there, instead of a hot aluminum spark plug seat, they use a few kilograms of Pu-239, which has so much alpha particle emission that it stays plenty hot, even in the vacuum of space. The Pu-239 is mounted on the far end of a boom to keep it away from the sensitive instruments of the probe.

That's exactly what it is, a thermocouple. It generates a very small amount of current, which the gauge reads. No external power needed.

--DD