What coolant switch/sensor is on the hose running to the expansion tank?
 

I removed a sensor/switch on the hose running to the expansion tank and I'm trying to figure out what it does. It looks like a coolant sensor, but I believe that is on the engine block (above the water pump). This part has a part number 951-606-106-00. A google search shows "temperature switch". Would this affect my temperature gauge that acts twitchy? Would is cause a rough idle during start up? I already replaced the other temperature sensors and the gauge still acts the same and my idle is rough at startup. I am currently cleaning/rebuilding my throttle body.
Thanks

That is the temp sensor for the turbo waterpump. It will not have an effect on how the car runs or gauge readings. It simply controls how long the waterpump stays on based on heat in turbo center section.

warning: if you disconnect that, it will contribute to premature turbo failure.

Mine broke off where the plastic part of it is, back when I replaced the head gasket. I was able to glue it back together, but how would I actually know if it still works? I did change the capacitor in the cooling pump relay for double the run time (1 minute after reaching "cold" temp).

If I let the engine heat up to the point where the fans kick on, then turn off the ignition, the cooling pump should run longer than a minute, correct?

I don't mean to sound like I am twittering, but I just got back from dunkin doughnut, where I got a sausage, egg, and cheese bagel, and a coffee - ahh, their coffee is soo good. Anyways, I let the car idle until the temp went up. But I don't think I heard the fans come on, because it started going down again, so I shut it off and started the stopwatch. The pump ran for 2:05, so the temp switch seems to be working OK, right?

Sounds about right. As long as it runs and shuts off it's fine. How long it runs depends on how hot the water jacket is on the turbo.

You can always test the pump by grounding the sensor plug to the block.

With all due respect, I'm going to have to disagree with that. If the temp sensor is bad, the pump may still run and shut off, but will only run for 30 seconds, no matter how hot the coolant is.