How would you check this?

I pulled the thermostat housing and instead of water, i found foam.

__________________
With best regards

Al

maybe I am wrong but I think if I pull the 100C switch coolant would pour out of the hole and I still don't I'd need to test the switch if it works or not.

If the piston is at the very top of the housing (can't remember now) you should be able to pull it. But if you don't see much water, then it's probably why it's not working correctly.

__________________
With best regards

Al